Pass distance metric from eye tracker to display

The on screen display now requires distance in addition to x, y as part
of its coords param. Update the eye tracking input driver to comply.
Merge pull request #2 from casebeer/fix/missing-distance-in-eye-tracker-display

Pass distance metric from eye tracker to display
Remove debugging stuff in arduino_serial

Debugging code is breaking arduino_serial due to hardcoded output file
names.

Remove.
Restore serial_output driver functionality

The serial_output driver for Arduino mouse emulator support was losing
data somewhere between sending mouse coords into the OpenCV/parent
process' end of the shared pipe and receiving that data in the
serial/child process' handler.

Reimplement the multiprocessing handler and change the shared data
strucutre from a Pipe to a multiprocessing.Queue.

This lets us

1. block while waiting for data to become available on the queue, and
2. block while sleeping/waiting for the next mouse output update
interval to trigger.