add two methods to the undomanager

Expose two methods on the undo manager class, `isClean` and `markClean`. These don't show up in the html docs, but can be seen here: https://github.com/ajaxorg/ace/blob/master/lib/ace/undomanager.js#L152

Cheers!
Ah, good catch; changed to `_proxy['markClean'] = markClean`.