Add ability for user to make moves.

We have added the ability for the user to make moves on the board. They
can now click on a number, and if the number have a space to move to,
the number will move to the open space. The following changes necessary
to add this feature are:

  - Added getIndices function to SlidingPuzzleView.

  - Added poll for pygame event MOUSEBUTTONDOWN in
    SlidingPuzzleController

  - Added moveNumber function to SlidingPuzzleModel

  - Added some member variables to SlidingPuzzleView to keep track of
    board width, board height, cell width, and cell height