Added undoAll() and redoAll() methods

Hello Oliver,

First of all, thanks for your work, which is very helpful for us.

My team and I have been using your undoManager for our application. We especially need to undo/ redo all the changes that have been tracked by an undo manager.
We cannot rely on magic fusion (which is really clever by the way) since changes can happen a different timings.

I guess we could use the undoTypes system to achieve this goal but it seemed more straight forward to implement the methods _undoAll()_ and _redoAll()_

I also added a test.
What do you think ? Would this feature be welcome in your API ?