adding media support via extensions

[jondot]

Hi!,
This should cover #8, I'm using it in production.

Sorry for no tests - looks like the existing libraries are too old and I don't have the time to set it up currently.
If someone can save me some time and let me know how to best run the tests on this one, I'd be happy to push a couple of tests in.

[jondot]

A word about using it, here:

    require 'atom/extensions/media'


    f.entries << Atom::Entry.new do |e|
       e.media_content = Media.new :url => article.media['image']
    end

[tjdett]

#23 may have fixed your testing issue, as it also updated the Bundler dependency to something more modern.