Pull Request: Add support for Stationary joystick Base

Added support for a stationary joystick base.  Changed virtualjoystick.js by adding some options to the 'opts' object.  These new parameters are boolean flag 'stationaryBase' and integers 'baseX' and 'baseY' , that are set when creating the initial joystick object.

Also added some simple branching to detect 'if(this._stationaryBase == true)' or not, and perform the appropriate behavior.  virtualjoystick.js will behave in the old way if the new 'stationaryBase' option is not used at all.