Add a text field that accepts a Github URL

Makes it easy to visit the site and paste a Github URL, avoiding the need to mess around in the URL bar.

![screen shot 2013-08-06 at 6 38 16 pm](https://f.cloud.github.com/assets/48019/921644/272f4358-ff02-11e2-9ac4-af850ea447d2.png)

With `pattern` validation magic (plus fallback for Safari). Tested in Chrome, FF and Safari on OS X.