Pull Request: Added Support for Limited Stick Travel

This Pull Request adds support for limiting the distance that the Joystick is allowed to move from its base.  By setting 'limitStickTravel' to 'true', it will limit the stick travel to a circle.  The size of this limit circle is set by setting 'stickRadius' to the desired number of pixels.  If no number is given, the limit defaults to 100 pixels.