new Relation API

as an effort to make Thing simpler and more
versatile , some functionality has been moved
to the brand new Relation API (decoupling a little the core things support):

-basicgame's relations are inteded for specifying
pseudo-mathematical relations between Things,
Things and the Stage, Thing and some position,etc.

-Now, collition detection, is interpreted as a
Relation (there is a class Collided that extends
Relation). And, the same is true for Crossability property.

have fun, hernan!