GamePad: Add Detect factory function.

Conflicts:
	include/bakge/input/GamePad.h
	src/input/GamePad.cpp
GamePad: No longer inherits Bindable
GamePad: Add Axes and API
GamePad: Uses Get/SetAxis instead of operator[]
GamePad: Removed SetAxis.
Taking control of a gamepad's input will be left to the end-user to
implement.
GamePad: Inlining GetAxis method
GamePad: GetAxis implementation.
GamePad: Fixing const compile errors.
GamePad: Detect method implementation.

Conflicts:
	test/gamepad.cpp
GamePad: Newline fix
GamePad: Add Buttons API
GamePad: Call Get* on Pad after creation
Initializes NumButtons/Axes.
Input: Add XBoxController header.
Contains enumerations of XBox controller axes and buttons.