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.
Replace file_get_contents() with Curl

- Replace file_get_contents() with Curl because Curl is faster and
  file_get_contents() is disabled on a lot of servers for security
  reasons.
- I also notice Curl getting a lot of results back from
  websites where file_get_contents() didn't work.
Merge branch 'feature/replace-file_get_contents-with-curl'