Support only_process option to process_in_background

Paperclip::Attachment.reprocess! doesn't use the only_process option
set by has_attached_file.

By default, reprocess! will delete and rewrite every style, including
:original, which is unnecessary if you're just creating thumbnails.
Add :only_process option to README
Merge pull request #21 from jakepaul/only_process

Support only_process option to process_in_background