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.
This error message is actually wrong.  The tumblr_oauth_token and tumblr_oauth_token_secret can only be provided via a Service.  Maybe you can fix where you copied this from too?