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 should be replaced by

```ruby
gem_dependency_check { defined?(Tumblr) }
```

(It's a Class method that takes a block.)