Fallback to use image_src if ogp::image isn't set

- Some websites (eg. imgur.com) don't have OGP tags but they do
  indicated an image associated with the page using image_src.
  Facebook's oEmbed falls back on this if OGP data isn't avalable.
Merge pull request #9 from MitchellMcKenna/feature/image_src-fallback

Fallback to use image_src if ogp::image isn't set