Added gamepad support using xinput.

[erstrom]

Hi, I have added Xbox360 controller support in FPS camera mode. Unfortunately I was unable to use the Unity Input class since the game only has two axes (Unity default axes "Horizontal" and "Vertical") defined. These axes are mapped to the left thumb stick of the controller, but I needed both and since adding new axes can't be done from code (as far as I understand) I decided to go for the XInput API instead.