added code to export keystroke and mouse data

currently all key/mouse events are dumped after the gaze block. That
is, the data are not sorted.

- could have created a table in HDF5 or in numpy and sort and export.
But given the way the data are handled in video2aoi.py, I am not going
to bother.

- time are zeroed at the first gaze event. Then any prior keyboard and
mosue events are ignored.