New provider: INWX (#808)

* adds initial support for INWX

* adds all features to the INWX provider

* allows domain for tests in creds.json for INWX

* runs go generate to update docs for INWX

* fixes formatting with gofmt

* changes goinwx to github.com/nrdcg/goinwx v0.8.0

* simplifies inwx sandbox check

* changes inwx unknown key error to a warning

* adds models.PostProcessRecords for inwx records

* replaces strings.TrimRight with [:-1] to remove final dot for inwx

* adds a comment about the domain creds.json key for the inwx provider

* removes warning for invalid creds.json keys in the inwx provider

* adds TOTP calculation support for inwx

* adds comments to inwxProvider

* improves INWX error messages

* adds additional documentation about the TOTP support for INWX

* adds inwx documentation

* bumps goinwx to 0.8.1 to fix the inwx API

docs/_providers/inwx.md

@@ -0,0 +1,99 @@
+---
+name: INWX
+layout: default
+jsId: INWX
+---
+
+# INWX
+
+INWX.de is a Berlin-based domain registrar.
+
+## Configuration
+In your `creds.json` file you must provide your INWX
+username and password:
+
+{% highlight json %}
+{
+  "inwx":{
+    "username": "yourUsername",
+    "password": "yourPassword"
+  }
+}
+{% endhighlight %}
+
+### Two factor authentication
+
+If two factor authentication has been enabled you will also need to provide a valid TOTP number.
+This can also be done
+via an environment variable:
+{% highlight json %}
+{
+  "inwx":{
+    "username": "yourUsername",
+    "password": "yourPassword",
+    "totp": "$INWX_TOTP"
+  }
+}
+{% endhighlight %}
+
+and then you can run
+
+{% highlight bash %}
+$ INWX_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 shown once when two factor authentication is enabled and you'll have to make sure to write it down then. 
+
+**Important Notes**:
+* Anyone with access to this `creds.json` file will have *full* access to your INWX account and will be able to transfer and/or delete your domains
+* Storing the shared secret together with the password weakens two factor authentication because both factors are stored in a single place.
+
+{% highlight json %}
+{
+  "inwx":{
+    "username": "yourUsername",
+    "password": "yourPassword",
+    "totp-key": "yourTOTPSharedSecret"
+  }
+}
+{% endhighlight %}
+
+
+### Sandbox
+You can optionally also specify sandbox with a value of 1 to
+redirect all requests to the sandbox API instead:
+{% highlight json %}
+{
+  "inwx":{
+    "username": "yourUsername",
+    "password": "yourPassword",
+    "sandbox": "1"
+  }
+}
+{% endhighlight %}
+
+If sandbox is omitted or set to any other value the production
+API will be used.
+
+
+## Metadata
+This provider does not recognize any special metadata fields unique to
+INWX.
+
+## Usage
+Example Javascript for `example.tld` registered with INWX
+and delegated to CloudFlare:
+
+{% highlight js %}
+var regInwx = NewRegistrar('inwx', 'INWX')
+var dnsCF = NewDnsProvider('cloudflare', 'CLOUDFLAREAPI')
+
+D("example.tld", regInwx, DnsProvider(dnsCF),
+    A("test","1.2.3.4")
+);
+
+{%endhighlight%}
+
+
+