Add support for GitHub Enterprise [Fixes #116]

This adds an option to provide a GitHub Enterprise URL (endpoint), which is used instead of the default (api.github.com). For example; the url for my local GitHub Enterprise instance was `http://192.168.1.115/api/v3` - so the description reflects that format.

![Screenshot](http://i.imgur.com/hC8mTSE.png)
The GitHub Enterprise API is accessed differently than GitHub.com. http://api.github.com is equivalent to http://github.local/api/v3 for Enterprise.

With this I have left the option open to use different versions of the API (seeing as not all GitHub Enterprise installations may be running the same version). Though I can see this not being entirely suitable for forwards compatibility anyway - for instance; the way we interact with the GitHub API may not work for newer versions.