Issue #8 Keeping originals

Added flag "original" allowing for keeping original image as well as storing resized and thumbnail images. Also supports .original in template.
Abstracted some classes to use with both original and thumbnail.
Filenames are the same pattern as with thumbnails excepting that .thumbnail. is replaced with .original.
No updates to doco yet. Will hopefully get round to that soon. Just an original = True flag passed to the field on the model.

This works in one of my projects:

    picture = StdImageField(upload_to='gallery/GalleryImages', blank=True, size=(640, 480), thumbnail_size=(100, 100), original = True)