SC.UndoManager: Added an option to automatically group the changes 

See the discussion here : https://github.com/learndot/sproutcore-wysiwyg/issues/22

Basically, this PR add a groupLapse property. See the comments in the code for more details.
If you can think of a better name, let me know (I'm not a native english speaker).

Note that SC.UndoManager has no unit tests yet and I don't really have time now to add them for scratch.