Gamepad fixes / polish.

Removing GamepadButton by adding _dataBinding member to Button.
Changing Gamepad::ButtonMapping enum values in order to remove getButtonMappingByIndex() which was a clunky and slow way of doing things.
Fixing crash in Game::shutdown() with Steve's fix.  (Thanks for catching that!)
Initializing UI pointers to NULL in Gamepad.
Correcting joystick normalization in Windows.
Updating Lua bindings.