Small patch.

Changes:
-Added missed Transform overloads to Vector2 and Vector3.
-Added arithmetical operators for Point This must significally make easy work with this struct.(I think XNA developers just forget add this to XNA).
-Removed useless GetState overload method at Keyboard(I know, I know this is for backward compablility - but its so easy to change for the end-user, because its primary used in just one place at Update).