Allow processing some styles in foreground, some in background
Allow processing some styles in foreground, some in background

Detect when only_process is set but different on has_attached_file and
process_in_background. In which case run foreground styles in normal hook,
and allow the backgrounded job to handle the styles on process_in_background
as normal.

owahab/delayed_paperclip,hewrin/delayed_paperclip,matt0903/delayed_paperclip,cgunther/delayed_paperclip,jrgifford/delayed_paperclip,leighhalliday/delayed_paperclip,ombulabs/delayed_paperclip