Add ability to store width/height/size in their own columns 

Hi,

I've made some changes which add the ability to store the metadata (width/height/size) in their own database columns (eg. User.avatar_thumb_width, User.avatar_height, User.avatar_original_size) as well as in the meta column.

This enables you to easily sort by width, height and size and thought it would be a useful addition to the paperclip-meta gem.

Regards,

--Richard Strand