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)
How comes this description has api/v3 in it?