DNS lookup of channel server

This is so @imaginator can run his own channel server and still make use of google apps.

Requires an SRV record as follows:

```
_buddycloud-server._tcp.imaginator.com.        IN SRV 5 0 5269 buddycloud.imaginator.com.
```

You can also set a similar record for a _topics_ domain.

Will run at the same time as a DISCO request and will often return much more quickly than a standard DISCO request. XMPP server still has to make standard s2s connection and security measures (not handled here).

Ready for merge and deploy, bumped version to 0.14.0.