NEW PROVIDER: HEDNS: Hurricane Electric DNS (dns.he.net) (#822)

* Add initial dns.he.net provider support

* Update to new IncrementalDiff interface

* Fix ListZones output for `all` query on `get-zones`

* Refactor authentication code for 2FA with better error checking

* Fix integration test and refactor zone record retrieval

* Add option to use `.hedns-session` file to store sessions between runs

* Add comment on `session-file-path`

* Add integration test for TXT records longer than 255 characters

* Add additional checks for expected responses, and better 2FA error checking

* Minor documentation changes

* Revert "Add integration test for TXT records longer than 255 characters"

This reverts commit 657272db

* Add note on provider fragility due to parsing the web-interface

* Resolve go lint issues

* Clarify security warnings in documentation

docs/_providers/hedns.md

@@ -0,0 +1,108 @@
+---
+name: Hurricane Electric DNS
+title: Hurricane Electric DNS Provider
+layout: default
+jsId: HEDNS
+---
+# Hurricane Electric DNS Provider
+
+## Important Note
+Hurricane Electric does not currently expose an official JSON or XML API, and as such, this provider interacts directly
+with the web interface. Because there is no officially supported API, this provider may cease to function if Hurricane
+Electric changes their interface, and you should be willing to accept this possibility before relying on this provider.
+
+## Configuration
+In your `creds.json` file you must provide your `dns.he.net` account username and password. These are the same username
+and password used to login to the [web interface]([https://dns.he.net]).
+
+{% highlight json %}
+{
+  "hedns":{
+    "username": "yourUsername",
+    "password": "yourPassword"
+  }
+}
+{% endhighlight %}
+
+### Two factor authentication
+
+If two-factor authentication has been enabled on your account you will also need to provide a valid TOTP code.
+This can also be done via an environment variable:
+
+{% highlight json %}
+{
+  "hedns":{
+    "username": "yourUsername",
+    "password": "yourPassword",
+    "totp": "$HEDNS_TOTP"
+  }
+}
+{% endhighlight %}
+
+and then you can run
+
+{% highlight bash %}
+$ HEDNS_TOTP=12345 dnscontrol preview
+{% endhighlight %}
+
+It is also possible to directly provide the shared TOTP secret using the key "totp-key" in `creds.json`. This secret is
+only available when first enabling two-factor authentication.
+
+**Security Warning**:
+* Anyone with access to this `creds.json` file will have *full* access to your Hurricane Electric account and will be 
+  able to modify and delete your DNS entries
+* Storing the shared secret together with the password weakens two factor authentication because both factors are stored
+  in a single place.
+
+{% highlight json %}
+{
+  "hedns":{
+    "username": "yourUsername",
+    "password": "yourPassword",
+    "totp-key": "yourTOTPSharedSecret"
+  }
+}
+{% endhighlight %}
+
+### Persistent Sessions
+
+Normally this provider will refresh authentication with each run of dnscontrol. This can lead to issues when using
+two-factor authentication if two runs occur within the time period of a single TOTP token (30 seconds), as reusing the
+same token is explicitly disallowed by RFC 6238 (TOTP).
+
+To work around this limitation, if multiple requests need to be made, the option `"session-file-path"` can be set in
+`creds.json`, which is the directory where a `.hedns-session` file will be created. This can be used to allow reuse of an
+existing session between runs, without the need to re-authenticate.
+
+This option is disabled by default when this key is not present, 
+
+**Security Warning**:
+* Anyone with access to this `.hedns-session` file will be able to use the existing session (until it expires) and have
+  *full* access to your Hurrican Electric account and will be able to modify and delete your DNS entries.
+* It should be stored in a location only trusted users can access.
+
+{% highlight json %}
+{
+  "hedns":{
+    "username": "yourUsername",
+    "password": "yourPassword",
+    "totp-key": "yourTOTPSharedSecret"
+    "session-file-path": "."
+  }
+}
+{% endhighlight %}
+
+
+## Metadata
+This provider does not recognize any special metadata fields unique to Hurricane Electric DNS.
+
+## Usage
+Example Javascript:
+
+{% highlight js %}
+var DNSIMPLE = NewDnsProvider("hedns", "HEDNS");
+
+D("example.tld", REG_DNSIMPLE, DnsProvider(HEDNS),
+    A("test","1.2.3.4")
+);
+{% endhighlight %}