BF: _onPygletKey was using clock.getTime

All other global timers, including what is used in _onPygletChar use
clock.getTime() (i.e. use reative to exp. start)
Merge branch 'master' of git://github.com/psychopy/psychopy
Merge pull request #571 from isolver/master

BF: _onPygletKey was using clock.getTime