Added 'Powered by Dropplets' display option

Just realized I should have probably made a new branch for this PR (Still new to actually contributing on Github - geeking out over being able to!)

Anyway... I really like what's going on w/ Dropplets and I think people should have an
option to promote Dropplets from their blogs.

Also like the way it's promoted [here](http://jason.sc/). So, I added an
option in settings, the extra image and css, and implemented the display
in the two themes that ship w/ Dropplets. It's disabled by default.