Three small fixes

There are three small bugs in the code as it stands:
  1. The randomization method allows for the same tile to be chosen repeatedly, e.g., ['boy.png', 'boy.png', 'boy.png', 'boy.png' ... ], 
  2. The tile type array is appended to itself, in order, so the matching tiles are always in the same column, two rows below (or above),
  3. The app behaves incorrectly if the player taps their active tile a second time rather than choosing a new tile.

This pull request fixes these issues.