Created a basic homebrew formula

(See [Issue 44](https://github.com/petervanderdoes/gitflow/issues/44))
Created a basic homebrew formula for review before submitting as a pull
request to the homebrew repository. This is almost entirely based on the
homebrew formula for nvie/gitflow.

The resulting homebrew formulae would be git-flow-avh and git-flow-completions-avh, respectively