Provide Boolean query method for attribute.

I've been using Dragonfly for years (after being a file_column, then paperclip refugee). It is exactly what I need. I finally got tired of typing the _uid part of my conditional tests. I.E.

    <%= image_tag obj.image.thumb('200x200').url if obj.image_uid? %>

With this patch it is now:

    <%= image_tag obj.image.thumb('200x200').url if obj.image? %>

Not a big change but helps make it just a bit more seemless. :smile: