Factor out saving a PIL Image to django storage.

Both the thumbnail and crop template tags do the same thing for taking a PIL
Image and saving it to a (potentially remote) storage. In addition, I need this
functionality in $SECRET_PROJECT, so I've factored it out into its own method.
Make CounterField increment by specified value instead of 1.

Add support for CounterField's increment and decrement functions to
increment/decrement by a specified value rather than 1. The values still
default to 1, so existing functionality continues to work.

Reviewed at http://reviews.reviewboard.org/r/3948/