Try to use web worker for motion detection

[szimek]

The motion detection works pretty slow and causes video to pause quite often. We could try out using web worker, send an image to it and perform the detection there.

[grechut]

I ran a bit of poor man's profiling..

So the current compare in motion detector took ~130s.
~75% of time was spent on comparison of images.
~25% of time was spent on taking a screenshot.
Current solution works on hidden canvas of size 400x300

After reducing size of hidden canvas to 100x100 (which still seems to detect motion quite well), I got ~10 times faster performance.

So maybe https://github.com/grechut/watchdog/pull/31/files is good enough, short-term fix.
Right now we're not saving any screenshots.
Once we return to the development of screenshots saving, we can move the work of motion detector to a web worker.
We can also work on its performance. Maybe there are better, faster motion detection algorithms.

[grechut]

Saw this nice snippet today: https://gist.github.com/SunboX/5849664
Might be helpful :)