Made modifications to reduce queries when using remote storage.

This should mostly fix issue #195. I also ran the tox test suite and all tests pass except for Python 2.5 tests. I mentioned that easy-thumbnails should drop 2.5 support in pull request #224.

Also, I'm not sure if another issue should be created but it would be nice if easy-thumbnails had an option to use Django's caching framework instead of the database.

- Made a modification to the Thumbnailer class so it doesn't make a query to the source table if it's already been queried once before.
- Updated the function generate_all_aliases so after all the thumbnails for all aliases have been created, it updates the modified time on all the thumbnails to match the modified time of the source file. This prevents the thumbnails getting recreated on first load.