NEW PROVIDER: deSEC (#725)

* Add initial deSEC support

* Handle the api rate limiting

* Fix deleteRR and do some code cleanup

* improve rate limiting and record deletion

* Add documentation for deSEC provider

* README.md update list of supported DNS providers

* deSEC supports SSHFP records

* dynamic minimum_ttl and hint for DNSSec on domain creation

* merge all changes into one single bulk api request

* Fix: actually set the TTL to min_ttl if necessary

* use a constant for apiBase URL

* Fix code comments

* Use PUT instead of PATCH for upsertRR method

* use ' instead of " for java script examples

docs/_providers/desec.md

@@ -0,0 +1,36 @@
+---
+name: deSEC
+title: deSEC Provider
+layout: default
+jsId: DESEC
+---
+# deSEC Provider
+## Configuration
+In your providers credentials file you must provide a deSEC account auth token:
+
+{% highlight json %}
+{
+  "desec": {
+    "auth-token": "your-deSEC-auth-token"
+  }
+}
+{% endhighlight %}
+
+## Metadata
+This provider does not recognize any special metadata fields unique to deSEC.
+
+## Usage
+Example Javascript:
+
+{% highlight js %}
+var REG_NONE = NewRegistrar('none', 'NONE');    // No registrar.
+var deSEC = NewDnsProvider('desec', 'DESEC');  // deSEC
+
+D('example.tld', REG_NONE, DnsProvider(deSEC),
+    A('test','1.2.3.4')
+);
+{% endhighlight %}
+
+## Activation
+DNSControl depends on a deSEC account auth token.
+This token can be obtained by logging in via the deSEC API: https://desec.readthedocs.io/en/latest/auth/account.html#log-in