Increased the number of arguments the apply function uses 'call' for from 3 to 4
Explained the way the ObjectRegistry registeres objects in a lengthy comment
Added a 'get' function to the ObjectRegistry to get a copy of the registered objects
The manager now triggers a Backbone-Event "undo" or "redo" after something was undone / redone
isAvailable("undo" / "redo") now returns wether there are actions in the stack to be undone / redone or not
Added merge function

The merge function enables to "merge" several undoManagers so that they
write on only one stack
Indented something for better legibility
Fixed bug in the ObjectRegistry's 'get' function that only returned objects which didn't had a cid