ce_content :: added save callback to youtube field to extract video id from a url

Quite a few users struggle with extraction of the video id from a youtube url. Especially with the recently added forced playlist.
This patch adds a save callback to the ce_content.youtube field that extracts the video id from a complete url. The user can just copy and paste whatever the address bar says.
You should probably set `'decodeEntities'=>true` in the eval array instead of manually decoding.