Added support to oEmbed #125

http://oembed.com/ #125 
Widht/height need to vary depending on a specific asciicast. The trick here is that the actual html widht/height can't be calculated server-side because it depends on font rendering of the browser. OEmbed spec says these 2 params are required, but we don't have a reliable way to calculate them. That's the main reason why there wasn't OEmbed support in Asciinema so far.