Add custom groups possibility

Implemented #262.

I've added an entry to the MainActivity menu "Manage Groups" that starts a new Activity where the user can make a new group, rename or delete an existing group. After deleting a group, the users of that group are automatically moved to "Friends" group.

The "Friends" group is the default group and it can't be modify. 

In the MainActivity one can choose to see only the members of a specific group. Also, the header is updated in correspondence with the group name.

The other activities that involve a possibility of working with groups are also updated: adding a friend, profile of a friend, accepting a friend request.

A friend can be moved to a different group by longclicking on his name and choose the option "Move to group".

Testing is very important now, so play with the groups if you can. I've tested with normal cases and corner cases that I've thought of and it works fine.