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.
Add a line at the top of the description like

```
      #{'## Include `tumblr_client` in your Gemfile to use this Agent!' if dependencies_missing?}
```

and define `dependencies_missing?` like

```ruby
    gem_dependency_check { defined?(Tumblr) }
```

(See twilio_agent.rb for an example.)