When removing an element, the 'ui_object_id' can not be set to the null object because the attributemap expects it to be a string. This change uses the string 'null' instead of the null object for the value.
Make use of the fact that 'elem.dataset' is a map, and just remove the item instead of inserting strings with special meanings. Also add some asserts for {clear,get,set}Index() methods.
Merge pull request #84 from tgmauch/master

Use Map nature of Element's dataset to manage attributes in ElementMapperImpl.