Added 'Powered by Dropplets' display option

Really like what's going on w/ Dropplets. 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.