Allow only_process through process_in_background

Hello, thanks for this gem. I'm still struggling to get it work with:
- Rails (2.3.18)
- Paperclip (2.7.5)
- DelayedJob (2.0.7)

Although this fix allows to pass the option `only_process` to the paperclip options there is another problematic issue. only_process is not working as intended...

What I'm really confused at the moment is the fact that on [paperclip implementation](https://github.com/thoughtbot/paperclip/blob/v2.7/lib/paperclip/attachment.rb#L122) it will only do the `only_process` if `post_processing` returns true. On your current [implementation](https://github.com/jrgifford/delayed_paperclip/blob/master/lib/delayed_paperclip/attachment.rb#L21) this always returns `false`. Is this a bug?