Remove pinry.pins.views.delete_pin

REST API should be used to delete pins anyway, so remove the old code
that is not being used.
Another major Pinry model rewrite

Generate thumbnail and standard image on request, and use
http://github.com/mirumee/django-images for generating them. Also,
remove the CreatePin page as pin creation is going to be done
in JavaScript. Create UploadImage view for uploading images from
computer.