fixed Vector2D bugs
Improve Collision onHit documentation

This commit fixes #739.
Improve attr documentation

This commit fixes #738, it also adds new signatures and more
detailed explanation to the inner workings of attr.
Merge pull request #741 from mucaho/recent_docs

Improve attr documentation
Merge pull request #734 from mucaho/fix_vector2d

Crafty.math.Vector2D improvements
Merge pull request #742 from mucaho/docs_onHit

Improve Collision onHit documentation