Fix "hasObject" in "group": no exception if empty string passed.
No nullp, use none instead. Further bool operators in ImplContainer.

Abandon use of "nullpointer" and use "boost::none" instead. Add further bool behavior to ImplContainer. Adapt all test cases too.
Tiny fixes to ImplContainer & Group.
Merge pull request #154 from balint42/fix_group_and_nullp

Fix group exception and nullpointer use