Post Images: grab images when HTML uses single quotes

Some external post editors insert images using single quotes, like so:

`<img src='http://example.com/image.jpg'>`

As a result, the images are not detected by `Jetpack_PostImages`, and do not get added as Open Graph Image tags. This commit allows you to use both single and double quotes.

Reported here:
* http://wordpress.org/support/topic/jetpack-not-publishing-photos-on-facebook-and-g?replies=7#post-5060473