Add rake task for reprocessing only meta data

The previous way of generating meta data for existing
Paperclip::Attachments was to reprocess them all.  For existing
implementations with lots of records and many styles that take time to
resize and process thumbnails that could be a lot of overkill.   This
creates a simple task that only generates the meta data for existing
attachments and their styles.
Even though it will work, the conflict between local variable name and method "meta" makes me nervous. Maybe "new_meta", "updated_meta" or something? And is it worth comparing new to old before writing it, or will ActiveRecord already accomplish that with its dirty checking?