Make next (obj) to remove code duplication

[Innoviox]

Currently Square.next and Tile.next are implemented the same. I propose a global next method, like so:

def next(obj):
if obj is a square or tile:
{default next implementation}
raise type error

Alternatively, a class "positionable" with a next method that both square and tile inherit from, but this seems a little weird.

[Innoviox]

Was fixed using the above implementation, but hasattr instead of isinstance. Also makes it easier to implement board attr checking later on.