Only use photon for accepted image types

If the URL doesn't end in one of the accepted image extensions, photon
will fail with a bad request error.
Fixed PocketAPI integration
Merge branch 'release/3.7' of github.com:wordpress-mobile/WordPress-iOS into release/3.7
Updated translations. Added Thai
Enable loading more content while syncing

Off by default. Subclasses must set a flag indicating they support this feature.

resolves #16