1.6.0: add event dispatchers for start/change/end interactions

- Also provide the values of the handles and indexes changed
- Add some tests for the dispatched events
- Update Readme.md

the Change event will also fire for keyboard interactions, but
currently I decided to avoid the "start" event for keyboard users
as it is kind of tricky to implement, and the "end" event is impossible
to implement. I may consider a focus/blur event at a later time.

resolves: #6