Add SoftLayer DNS provider (#59)

Add SoftLayer DNS as a DomainServiceProvider.

The SoftLayer API is a bit of a mess and treats MX and SRV records
differently. This leads to some replication and custom handling issues
to work around.

In this patch I have to change the SRV test case to be _tcp instead of
_protocol because softlayer requires a "known" protocol which AFAICT is
tcp, udp or tls. I think this will be acceptable in most cases.

Signed-off-by: Jamie Lennox <jamielennox@gmail.com>

docs/_providers/softlayer.md

@@ -0,0 +1,50 @@
+---
+name: "SoftLayer DNS"
+layout: default
+jsId: SOFTLAYER
+---
+
+# SoftLayer DNS Provider
+
+## Configuration
+
+To authenticate with softlayer requires at least a `username` and `api_key` for authentication.
+It can also optionally take a `timeout` and `endpoint_url` parameter however these are optional and will use standard defaults if not provided.
+These can be supplied via the standard 'creds.json' like so:
+{% highlight json %}
+    "softlayer": {
+        "username": "myusername",
+        "api_key": "mysecretapikey"
+    }
+{% endhighlight %}
+
+To maintain compatibility with existing softlayer CLI services these can also be provided by the `SL_USERNAME` and `SL_API_KEY` environment variables or specified in the ~/.softlayer.
+More information about these methods can be found at [the softlayer-go library documentation](https://github.com/softlayer/softlayer-go#sessions).
+
+## Usage
+
+Use this provider like any other DNS Provider:
+
+{% highlight js %}
+var registrar = NewRegistrar("none","NONE"); // no registrar
+var softlayer = NewDnsProvider("softlayer", "SOFTLAYER");
+
+D("example.tld", registrary, DnsProvider(softlayer),
+    A("test","1.2.3.4")
+);
+{%endhighlight%}
+
+## Metadata
+
+This provider does not recognize any special metadata fields unique to SoftLayer dns.
+For compatibility with the pre-generated NAMESERVER fields it's recommended to set the NS TTL to 86400 such as:
+
+{% highlight js %}
+D("example.tld", registrary, DnsProvider(softlayer),
+    {"ns_ttl": "86400"},
+
+    A("test","1.2.3.4")
+);
+{%endhighlight%}
+
+`ns_ttl` is a standard metadata field that applies to all providers.