Refs #75, adds conversion/scaling/cropping to post images.

Post images can now be uploaded as gifs, pngs, or jpgs, and will be converted to an 800x600 full size .png, a 400x400 medium thumbnail .jpg, and a 100x100 small thumbnail .jpg. These are the image_url, image_medium_url, and image_small_url attributes on the post object.