Allow multiple resources for the same JID in a MUC room

Implementation of esl/MongooseIM#240
I think these can be merged into `sessions_exist/2`, like:
```
sessions_exist(Nick, StateData) -> 
   case ?DICT:find(Nick, StateData#state.sessions) of
       {ok, [_Val | _]} -> true;
       _ -> false
   end.
```