Changed UI to print unicode chess pieces

Added a replace_pieces() function that replaces the text-based pieces with unicode chess pieces just before printing the UI. Option for light or dark terminal backgrounds.

![screen shot 2014-08-30 at 3 38 38 pm](https://cloud.githubusercontent.com/assets/2894907/4100419/6c2287e2-3096-11e4-839f-224e4c9869d7.png)