Add skeleton controller

Start of #11
Move the model member from View to Controller

Progress on #11
Add test todo
Split up View._handle_events and add tests
Finish splitting View into View/Controller

* Move the following from View to Controller:
** key constants
** _drop_x
** _print_controls
** the main loop
*** _handle_events
**** _quit
**** _attempt_to_drop_piece
**** _move
* Move the following from View to Model:
** current_player_piece
** end_turn

Fixes #11