Misc iohub BF and ENH

- Fixed issue with linux keyboard / mouse when pyglet 1.1.4 is used. Now the autogenerated x11 ctypes wrapper file that exists only in pyglet 1.2 has been copied to iohub/devices and pyglet is not imported at all by linux kb /mouse device.

- Latest GP3 eye tracker interface code

- Computer process priority changing related methods now catch AccessDenied exceptions from psutil and now return bool indicating if requested priority change occurred without exception or not via bool return value.

- Fixed bug in keyboard test script.

- Pull Req. includes start of psychopy api docs section for iohub. Doc changes can be ignored since they are incomplete if that makes most sense.