Register mediators

Closes #147 

@rcrichton if you could please review?
Yeah, I was also worried about overriding any changes someone may have made to the existing channel. To me the 'running' channel trumps default config; but then again a newer version of a mediator may want changes to the channel config...

Maybe we should treat updates with a different field in the registration message? e.g.
```
{
  defaultChannelConfig: [ ... ] //Only applied for new mediators. Must contain full channel info
  channelConfigUpdate: [ ... ] //Applied if version incremented. May only contain subset of fields
}
```

So at the end of the day, defaultChannelConfig equates to a mongo insert while config updates equate to a mongo update?