re-enable video playback

* remove old mediaelement.js styles since we only use its API
* add _video.scss for styling the current video element
* rename `.podlovewebplayer_top` to `.video-pane`
* refactor event play button creation and event listeners in app module
* reuse play button for video playback

fixes https://trello.com/c/Bd0eeOgd/78-fix-video-player
add poster image to video example