Can delete pictures which are not user pictures

Issue Summary

This module can delete pictures which are not user pictures. For example, if an avatar from the avatar_selection module is in use, and a new user picture is uploaded, the unique_avatar module will delete the avatar from the avatar_selection pool, which is not the intended behavior.

The attached patch makes unique_avatar check to see that the old picture is actually in the pictures directory before deleting it or clearing the image_cache cache.