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.
I see now why you added this, but it should be safe to remove.  `upgrade_warning` is only needed for the older Agents that used a different style of services.