able to specify paperclip format per style

Paperclip allow to specify style format : 
```ruby
 :styles => { :thumb => ["100x100>", :jpg] }
```
I was not too sure how to implement this using our current image settings aka
```ruby
Spree::Config[:attachment_styles]
```

Don't know if this is the right approach but I endup with a split(', ')
![spree-paperclip-style-format](https://f.cloud.github.com/assets/159814/376266/dcd79dcc-a417-11e2-9f9e-f53c098cf2a4.jpg)