improved new tile placement behavior
moved randomGrid() to grid.py
added tileScore, tileIterator and score methods
moved logic from main.py to game.py
added __repr__
added copy methods
added AI
implemented deck of coming tiles
(i.e. the supposed behavior in the actual game)
(see http://blog.braceyourselfgames.com/post/76156777645/threes-strategies)