Fixes --githubHost option for use with Github Enterprise.

Hello, sweet project.

When I try to compile a book using the `--githubHost` option my value isn't acknowledged:

![screen shot 2014-04-07 at 1 11 04 pm](https://cloud.githubusercontent.com/assets/461249/2634267/a5d063fa-be78-11e3-99fe-8a5d9ba582be.png)

Furthermore, if you try to compile a book using the shorter `-gh` option gitbook errors out:

![screen shot 2014-04-07 at 1 20 40 pm](https://cloud.githubusercontent.com/assets/461249/2634296/fce0cfe0-be78-11e3-8084-fe7adbb39174.png)

This PR attempts to the resolve the issue(s).

The `--githubHost` option wasn't being passed to the generator. After passing that along it works:

![screen shot 2014-04-07 at 1 26 19 pm](https://cloud.githubusercontent.com/assets/461249/2634366/c692af70-be79-11e3-9304-0ad9751ab89d.png)


 However, the shorter `-gh` still doesn't work correctly. I am not sure if it's an issue with commander and defining options for both `-g` and `-gh` or something else. It works when renamed to `-e` for example. Anyways, I just removed the `-gh` option in favor of the longer `--githubHost` similar to the `--theme` option. 

Please review and let me know what you guys think or if you'd like additional information/changes.