ENH: ioSync now supports analog input thresholds

Each analog input channel can have a threshold set. If the analog input
goes above or below the threshold level for that channel, a
ThresholdEvent is created.

Can be used to set voice key or light key 'triggering' sensitivity.
Merge branch 'master' of git://github.com/psychopy/psychopy
Merge pull request #575 from isolver/master

ENH: ioSync support for analog input thresholds