Support SRV records

As defined in [RFC2782](http://www.ietf.org/rfc/rfc2782.txt)

----

Example:

Declaring that service 'myservice' is available on port 8080 of node1.myservice.orls.net:

```shell
curl http://127.0.0.1:4001/v2/keys/net/orls/_tcp/_myservice/.SRV/whatever1 -XPUT -d value="100\t100\t8080\tnode1.myservice.orls.net"
```

Would allow the following:

```shell
$ dig @localhost _myservice._tcp.orls.net. SRV +short
100 100 8080 node1.myservice.orls.net.
```