Adds support for GitHub Enterprise.

Fixes #32

This tool kicks ass. I used it for back-filling tags in one of our projects.. 56 releases. I couldn't imagine trying to do that by hand!

```bash
$ github-changes --host github.fit.edu --path-prefix /api/v3 -o levilewis -r FIT-PHP-Namespace -k [sometoken] --use-commit-body -v --no-merges --only-pulls -f ~/Desktop/CHANGELOG.md
```

```BASH
fetching commits
pulled commit data for tag -  0.11.11
....
pulled commit data for tag -  0.8.0
fetched all commits
```

Sample of output file:
![screen shot 2014-05-05 at 1 19 02 pm](https://cloud.githubusercontent.com/assets/461249/2880661/69e8c186-d479-11e3-803e-ccbbc33f5c32.png)

Let me know if you want changes to the PR. Thanks for noticing and sorry I forgot to submit. This PR requires two of the dependencies to be bumped. So, please review for any potential issues before merging :beers: