feat($goConnection): Expose goinstant connection 

- goConnect has been retired as promises in core makes it redundant
- goUsers has been retired as promises in core makes it redundant
- The user model and factory have been retired
- Connection has been added, it's a very simple wrapper around connect
that makes configuration via. Angular possible
- goConnection is also a registered service, it can be used to access
anything normally available on the connection object

Previously Code Reviewed: https://github.com/goinstant/goangular/pull/22

Note: Commits will be squashed before merge.