fixed frame and time thumbnail capture

Turns out the actual video frame rate isn’t available until the capture
session is over (duh) so I moved the logic to calculate the CMTimes
just prior to the thumbnail creation. I also sort the times so multiple
thumbnails are in their expected order.
moved thumbnail generation to own method

also now respect thumbnaulEnabled flag; if YES then default thumbnails
are created for first and last frames of video
Merge pull request #165 from blushmessenger/master

fixed bugs in thumbnail creation; cleaned up code