added undo state change event class
added undo state changed event type to EventType
added undo/redo accessors and methods to Model
added undo state change event stream to Model
added test for undo
used array notation for js fields
moved canUndo/Redo up to js fields section
put event instance in alphabetical order
moved variable declaration inside test call
fixed closing bracket indentation
use type and more similar name for subscription variable
Merge pull request #3 from sirctseb/master

Support for undo/redo feature