This branch adds a plugin to expose the datapixx/viewpixx digital output functionality.

[cnuahs]

Note, this change touches the mcc plugin, so it would be good to get those using the mcc device/plugin to test and verify that these changes don't break their rig.

Summary:

• created plugins.datapixx to expose the digital output functionality (this is basically a wrapper around the psychtoolbox Datapixx() function calls).
• provides an interface to set/clear DOUT bits on datapixx/viewpixx devices.
• provides a means of setting up a frame synchronous "trial bit" that strobes high on the first frame of each trial.
• created a new abstract @daq class that abstracts the interface common to most daq devices (datapixx and mcc at this point, see note above).
• the mcc plugin now also derives from this base class.