Closes #28 - Add RFC for Homebrew Provider

This pull request includes the RFC brought up in #28.

This is a breaking change we'd like to get into Chef 12.

#### Homebrew As OS X Default Package Provider

* **Breaks Backwards Compatibility? (Workstation):** No
* **Breaks Backwards Compatibility? (Cookbooks):**  Yes
* **User Benefit:** High
  * Short description of the benefit: Homebrew is commonly used by members of the Chef community as a "package system" instead of Macports.
* **Impact:** Medium
  * Short description of the impact: This is a backwards incompatible change. It's unlikely that existing cookbooks would need to be rewritten if the package names are the same.
* **Objections:** The most common objection is that the homebrew cookbook would still be required to get homebrew installed, since it doesn't come with OS X. However, the same is true of macports (it has a cookbook to install as well).