Auto Video Embeding

This enables auto embedding of video links for YouTube, Vimeo and Dailymotion, only works with JS enabled.

Searches for links it can convert to embed tags and fetches the video preview image.  The image is added to the post with a single click to do the embed and autoplay and a double click to do the embed but not autoplay.

Vimeo and Dailymotion require an extra api request to get the link for the full size preview image, the image will be added as the response if fulfilled without any load blocking.

Works in posts and PM, works in previewing of posts and PM, works with quick edit (re embeds the video after editing)  does **not** "work" in WYSIWYG as I did not bother with doing that.

This is in response to #215 and http://www.elkarte.net/index.php?topic=211.0