* Adds a `picture` attribute to `User`.
* Preserves Gravatar as a fallback option.
* Now, you should use `user.profile_picturer` instead of `gravatar_url`
* TODO: Ensure `profile_picture` handles sizing right for uploaded images.
The single query was producing a tremendously large ActiveRecord allocation.
While this isn't nearly as cool as one query, it's still a significant
reduction in SQL trips, and now has the bonus of only loading what is
actually going to be displayed.
