TumblrPublishAgent

First attempt at a publish agent for Tumblr. Based very closely off of the TwitterPublishAgent. I'd like some feedback on the structure for inputs as well as the code structure before I start working on additional Tumblr-related agents.
Hmm. I've just been merging in upstream/master so that it can be auto-merged. Maybe I did something by accident. Looks like I should be able to roll back that commit and correct it, though.