BF and ENH: see description please.

BF: Removed debugging print.
ENH: Added keyboard modifier reporting for mouse events.
ENH: Added class attribute to ioHubConnection that gives the currently
active ioHubConnection so multiple connections / ioHub Server instances
are not created by accident within the same python session.
Merge branch 'master' of git://github.com/psychopy/psychopy
Merge pull request #320 from isolver/master

BF / ENH: removed print, added modifiers to mouse events, +