Refactor Imgur class to Imgur::Client
Add Imgur::Upload

* Add test coverage and `rake test` task.
Return an Imgur::Upload from Imgur::Client#upload
Replace Upload#error? with Upload#successful?

* Depend on rake >= 10.1.0.
* Depend on test-unit ~> 2.5.5.
Add Imgur::Notification
Add defaults to notifications
Use Imgur::Notification to send notifications
Add wip integration tests
Begin rewriting Alfred workflow script
Refactor multi-upload functionality into Imgur::Batch

* Prompt user to re-authorize with Imgur if the stored access token
  is invalid.