Allow storing of an image of a speaker.

Previously the only image for a speaker was just stored as a URL.
Add a model field to store a local image. This is stored using an
easy-thumbnails ThumbnailerImageField, so that we can later start
to use it to get thumbnail sized cropped images.
When saving a Speaker, set image_cache from image.

When a speaker is saved, cache the image at image_cache so that we can
use it for thumbnails etc. Store thumbnails on a per-instance basis.
Thanks to Matthew for test fixing.
Merge branch 'issues/363-avatar-sizing'