Support SOA record type

Example
```json
{
    "google.com" : [
        {
            "rdata": "62.252.169.178"
        },
        {
            "rtype" : "SOA",
            "rdata": {
	            "mname": "ns.google.com",
	            "rname": "dns-admin.google.com",
	            "serial": 2003080800,
	            "refresh": 172800,
	            "expire": 1209600,
	            "retry": 900,
	            "minimum": 3600
	        }
        }
    ]
}
```
Implements #37