Resizing images on the media model (Issue #179)

Resolves https://github.com/18F/peace-corps-product-management/issues/179

As with the SirTrevor image processor, creates and saves resized copies of any images uploaded to the Media model. Currently saving to the same place Sir Trevor saves--I'll be removing this functionality from Sir Trevor's processor once its images are being properly saved to the media model.

Question: This is currently pulling filenames for the resized images from the title field. Any preference to generate filenames a different way?