Add note on the future package

Simplify files structure

Drop unused imports

Use button group to simplify buttons management and event handling

Simplify close event

Make the code compatible with Python 2 and 3, PyQT 4 and 5

Fix issue with app still confirming quitting after game ended

When quitting, check if the game is still running and confirm with user

Add some info here and there

Merge pull request #3 from alexei/feature-separate_engine_and_ui

Separate game logic and UI

Refactor methods

Drop unnecesary attributes; rename value attr to position

Fix import

Merge pull request #2 from alexei/feature-simplify_buttons_ui