Vanity url

- [x] Ready for merge

This allows (pro) users to have rem.jsbin.com/foo and foo will only load if it was created by rem and they're a pro user, otherwise it redirects.

Share also includes their vanity url:

![screen shot 2014-03-24 at 17 55 14](https://f.cloud.github.com/assets/13700/2502995/8bbd901a-b37d-11e3-8e21-90b781b4ecac.png)

Todo:

- [x] Fix emberjs.jsbin.com - currently redriecting to jsbin.com on output urls
- [x] Ensure orgs work correctly
- [x] "Publish to vanity url" see option 3 in #1316