Run initial `bridge.Add` in a goroutine

The following just happened to me:

* A service was set to be registered at startup.
* `bridge.Add()` was called at the line in this patch
* However, there was a problem with registration, so initialization was stalled in (retry mode)[https://github.com/progrium/registrator/blob/51b069973a619a1a2eaa014c3665911837346ca9/bridge.go#L157].
* Thus init stalled, the docker api channel was never opened, and it looked like registrator was having problems listening to Docker events.