Add overrideBuiltInFunction

Signed-off-by: David Brainard <brainard@psych.upenn.edu>
Merge pull request #212 from kleinerm/master

PTB BETA RELEASE "Random acts of kindness" Psychtoolbox-3.0.12 - SP1

* Blacklist 'iSight' and 'Apple IR Receiver' as default keyboard for Linux keyboard queues.
* Allow configurable vtotal vs. vactive in Screen(), add setup code to loosen constraints for VPixx + Stereo.
* Update some help texts.
* Update PsychtoolboxRegistration to work with new registration server URL.
* Fix timestamp overflow for sending events with NetStation() to EGI Netstation system.
Merge pull request #50 from Psychtoolbox-3/master

Resync with current master of official Psychtoolbox-3 repo.