Dynamic only_process support

Enables lambda support for the only_process option to determine which
styles need to be processed delayed dynamically. This is supported in the generic paperclip gem, adding it to the delayed_paperclip gem allows you to process certain styles on attachments that support multiple file types as well.

```
process_in_background :avatar, :only_process => lambda { |a| a.instance.small_supported? ? [:small, :large] : [:large] }
```