disentangle FrozenBindings and FrozenDict
fix for python 2.5 compatibility problem

FrozenDict is based on Mapping which was only added to collections in
2.6. compat.py includes some compatibility code to base it on
MutableMapping, basing on DictMixin from UserDict, which is an old-style
class causing super not to work properly :-/
mixed up FrozenBindings and FrozenDict

that's what super is for, isn't it :D
Merge pull request #14 from joernhees/disentangleFrozenDict

disentangle FrozenBindings and FrozenDict