New module: YoutubeEmbed converts Youtube embed tags into screenshots that link to at link to videos

For example,

    <iframe width="853" height="480" src="http://www.youtube.com/embed/7aEYoP5-duY" frameborder="0" allowfullscreen></iframe>

becomes

[![Link to Youtube video](http://img4.glowfoto.com/images/2013/02/21-0820532358L.png)](http://www.youtube.com/watch?v=7aEYoP5-duY)

This is accomplished by:

- identifying the Youtube video url
- getting the screenshot from `img.youtube.com`
- layering the image with a play button using PIL
- uploading the resulting image to glowfoto (I chose glowfoto as the host because it is very quick and does not require any API key)
- generating markdown of the form `[![caption](image_url)](video_url)`

I've found this module to be useful because Github does not support embed tags, but it's still important to be able to post links to videos in readme files.