`brew` command now ran as user owning executable

This is a fix for https://github.com/opscode/chef/issues/1934.  Homebrew_owner now tries to run homebrew as the user who owns the `brew` executable, or an optional homebrew_package resource attribute.

If `brew` isn't at the default location (/usr/local/bin) we search for it with `which`.  If it cannot be found and the homebrew_package resource is missing, we raise an error.
https://github.com/opscode-cookbooks/homebrew/blob/master/libraries/homebrew_mixin.rb#L31-L39

The homebrew cookbook doesn't allow root installs at all.  The current homebrew package provider would allow root usage if the user specified `homebrew_owner 'root'` in the resource.

But now that I see the cookbook doesn't allow root installs, I wonder if we should allow using brew as root.