Introduce `TKTransition` model and integrate support for `userInfo`. closes #4

* Introduces a new `TKTransition` model that is passed to block callbacks describing the transition taking place
* Adds support for `userInfo` to event firing. The `userInfo` is passed with the transition and into notifications
* Add test coverage for new functionality
BumpIntroduce `TKTransition` model and integrate support for `userInfo`. closes #4

* Introduces a new `TKTransition` model that is passed to block callbacks describing the transition taking place
* Adds support for `userInfo` to event firing. The `userInfo` is passed with the transition and into notifications
* Add test coverage for new functionality
Bump version to 2.0.0
Update README.md for 2.0.0