Improvements for bbox, game, vector and sprite.

###### Bounding Box
1. New method for returning the y-value of the top border
2. New method for returning the y-value of the bottom border

###### Game
1. Allow existing `canvas` to be passed as a parameter (e.g., when it has been defined in HTML)

###### Vector
1. New method for directly setting x and y
2. New methods for performing division on the vector

###### Sprite
1. Fixed a bug where `@texture.onload` would only be called once for the last image that was added