Proposal: new SC.AutoGroupUndoManager class

This PR is based on this one : https://github.com/sproutcore/sproutcore/pull/1048

It extends SC.UndoManager to easily allow to group undo events by time. 
This may be used in sproutcore-wysiwyg. 

Let me know if you can think of a better name for this class or for the `groupLapse` property.