ENH: MovieStim2 beta (RC?) and demo


MovieStim2 should now work in terms of all public methods and input args. More help testing is always appreciated. ;)

* Looping is working now.
* Demo gives assert failure if video path is not filled in. Is there a video that ships with the demos we could use as the default?
* Demo shows how to use start, stop, pause, seek, and volume up and down.
* Jeremy, I tried 3 videos when running demo script within PsychoPy Coder,
and all seemed to work fine. Please let me know if u see issues.
* Not 100% happy with audio / video syncing all the time (it is not always
within 1 frame synced), but seems quite usable from the 10 videos have have
tried. More dev. time could be spent improving the A/V syncing, but I am out of
time for this right now.
* One video I have is corrupt because of no key frames (as informed by
text put to stderr from openCV). It plays, but pausing or seeking makes
it all messed up. Playing the video using VLC also shows issue; video
play position can not be changed with slider gui; and instead the movie
restarts each time slider is moved by user. So I think this is a genuine
video file issue.

Thanks.