Install hub via brew and alias g to hub

[Hub](https://github.com/github/hub) is a great way to work with GitHub!
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Most useful features are:

 - Use `user/repo` syntax everywhere to refer to GitHub URLs.
 - Create forks.
 - Work with multiple remotes for upstream and origin.
 - Create pull requests.
 - Apply pull requests.

It is great for both open-source contributors and maintainers.
See the different workflows here: http://hub.github.com/

**Yes, it is created and maintained by GitHub!**

It is recommended to alias `git` to `hub`, because it also enhances existing commands like `apply`, `am`, `clone`, `fetch` and others.

I am aliasing `g` to `hub` so when you type you will use `hub`, but other tools which use `git` will not interfere with `hub` abilities.