Support Enumerator as an input source

The Enumerator class can be used to stream data without having to actually write a dedicated class for it. For example:

enum = Enumerator.new do |yielder| 
  HTTPClient.get('http://some-website.com/some-big-file.xml') do |chunk| 
    yielder << chunk
  end
end

document = Oga.parse_xml(enum)