add gem source handler

I just started using fpm-cookery and would like a uniform interface to building packages. Since I use fpm to build Ruby gems, I hacked this branch together.

I don't know if this is of any value to you. Here's an example recipe:

```
class RubygemJson173 < FPM::Cookery::Recipe
  description 'This is a JSON implementation as a Ruby extension in C.'

  name 'rubygem-json'
  version '1:1.7.3'
  revision 1
  homepage 'http://www.rubygems.org/'
  source 'http://rubygems.org', :with => :gem, :name => 'json', :version => '1.7.3', :as => 'json-1.7.3.gem'
  sha256 '1070e6cd2fedc0cb06e7e56a980b884bdf6e61a9184abb1c6e677d308b4c26d9'

  section 'ruby'

  def build
    # noop
  end

  def install
    system "gem install --ignore-dependencies --no-rdoc --no-ri --bindir #{destdir}/usr/bin --install-dir=#{destdir}/usr/lib/ruby/gems/1.8 json-1.7.3.gem"
  end
end
```