Buddypress support

This is the first draft of adding buddypress group support to p2p. 

There is an issue which needs to be reviewed.,
1. All connections are automatically added to the dropdown on page load.

Additional features that need to be added 
1. Allow group parent to be defined so only sub_groups are listed (would require BP Hierarchy to be installed)
2. Filter groups by if user is a member
3. Filter groups by if user is an admin
4. Filter groups by status

All these option should be defined in connection registration
You should move this class to a separate file (call it `bp-group-query.php`, so that it gets loaded on-demand automatically).