Modified the AnnotateModels module so it only updates models that have changed since the last time 'annotate' was run. This is huge for those of us who like to run 'annotate' frequently but don't want to update our VCS for all the models in our application each time we do.

This required a few small modifications to the annotate_one_file, annotate, and do_annotations methods. Comments have been updated to reflect changes.