Feature/cache image dimensions locally

This is still early stage I feel as I'm sure there are cases I've missed, but anyway.. here we go:

Here we have a first attempt at caching the image dimensions for a particular thumbnail in the database.

As documented and also mentioned here: https://github.com/SmileyChris/easy-thumbnails/issues/293 it can be a very slow process to fetch the height and width of a thumbnail that is stored on a remote backend.

This hopes to address that!

My basic tests are working so far, I'm going to test on a larger set of examples as well.

Thoughts/criticisms etc?