Reworked reservations and rooms a bit

* reservation_rooms table now stores an additional field: configuration
 * Holds the configuration of the room (i.e. whether it is used as a single-,
   double-room, etc...)
 * Reworked the base model a bit to better support such relations
* Room cost columns are now floats
* Added bill projection in edit-reservation controller
Merge pull request #22 from DeX3/qa

Reworked reservations and rooms a bit