Support Users Directly Uploading Profile Pictures.

* 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.

app/views/users/show.html.haml

@@ -3,7 +3,7 @@
     .col-md-12
       .page-header
         %h1
-          = image_tag(@user.gravatar_url(size: '48'), title: "Yo #{@user.name}!", alt: '')
+          = image_tag(@user.profile_picture(size: '48'), title: "Yo #{@user.name}!", alt: '')
           = @user.name
           %small
             = @user.nickname