Added small DSL for formulas

You can now use "url", "version", "homepage", and any checksums as
class methods, like so:

    class Dos2unix <Formula
      url 'http://www.sfr-fresh.com/linux/misc/old/dos2unix-3.1.tar.gz'
      md5 '25ff56bab202de63ea6f6c211c416e96'
      homepage 'http://www.sfr-fresh.com/linux/misc/'
    end

The previous usage ("@url = 'http://example.com/tarball.tgz'") still
works, maintaining compatibility with previous formulas.

"dos2unix.rb" is translated into the new format as an example.