Add documentation on installing through Homebrew

This adds documentation about installing using Homebrew as suggested in #227.

I'm not 100% sure on the copy, please let me know if you have any suggestions on how to improve it.

This is what it looks like rendered:

![install packer - packer 2013-08-03 16-49-01](https://f.cloud.github.com/assets/24421/906371/46ad9b34-fc97-11e2-9cd0-3470f6197b9c.png)