Added code documentation for Keys.cs, KeyboardState.cs and Keyboard.cs.

* Added code documentation for Keys.cs, KeyboardState.cs and Keyboard.cs.
* The documentation is based on http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.input.keyboardstate(v=xnagamestudio.40).aspx, http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.input.keyboard(v=xnagamestudio.40).aspx, http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.input.keys(v=xnagamestudio.40).aspx but **I've worded them on my own**