Feature/oembed support

Hi @remy!

This is a first attempt to implement oEmbed support for JSBin. oEmbed allows you to embed rich content without running arbitrary code (unlike the current embed method) and therefore is the prefer method for a lot of people including Gitter, here some more info: http://oembed.com/ and some examples: https://embed.spotify.com/oembed/?url=http://open.spotify.com/track/4fyHcNgv4PizcRMXUGPFQ2

Here's how they would look:
![](http://i.imgur.com/lEmgNqc.png)

What do you think?
We haven't got a helper to validate, but to ensure this runs just as well on local installs could you check against `app.get('url host')` this will return `"jsbin.com"` or `"localhost:3000"` etc...