BF and ENH: ioHub related

Submitting this pull request to get the slate clean before committing the experimental movie2 stim.

See commit notes for details on  made; mainly eyetracker related ENH and BF's.

NEW: Added iohub Serial device. Has a simple serial input stream parser that can be used to define the content conditions needed to generate serial input events. Should also work with the PST serial response box.