Fix positioning of overlay content
Before, the content could be pushed too low when the video had a % height. Essentials had this issue - see https://blood/essentials/work-{name}/www/account/course/essentials/2012/lecture/opening

This needs to best tested across our various video sites but should work.
replace instances of childnodes[0] with firstChild
user margin-top instead of padding-top or else the size of the content is calculated with the padding and jumps when the window is resized
Merge pull request #28 from nburka/fix-video-overlay-positioning

Fix positioning of overlay content