Add support for GitHub Enterprise links

Hey there,

I really like this plugin but it doesn't work with GitHub Enterprise links.

To avoid having the user identify their enterprise url, this code searches for the "View it on GitHub Enterprise" link in the signature of GitHub notifications.

![view_it_on_enterprise](http://skitch.ubermajestix.com/view_it_on_enterprise-20131209-135408.png)

It then stores the Enterprise url it finds in a cookie so we can search for links matching this url later. Also, not all GitHub Enterprise notifications come with the signature, notably pull request notifications. Having the Enterprise url stored will allow us to identify GitHub Enterprise links.