DEMO: mouse.py spot follows the mouse

(and reveals a current bug: fails to follow after one click)
Merge branch 'master' of git://github.com/psychopy/psychopy

* 'master' of git://github.com/psychopy/psychopy:
  BF: more agressive forcing of updates (fixes strange bug in setting of stim pos)
Merge pull request #494 from jeremygray/master

DEMO mouse.py dynamic position