Added :tap / :untap actions to brew provider

Example usage: subversion17 # recipe to install svn 1.7

    unless /version 1\.7/ =~ `svn --version`
      brew 'homebrew/versions' do
        action :tap
        only_if "brew info subversion17 2>&1 | grep 'No available formula'"
      end
      brew 'subversion17'
    end