Cbache/creation of dict config getter

DictConfigGetter behave like ConfigGetter, except it can also be initialize with dictionary. We use this for testing purpose.
@dzen 
@rbarrois 
"""Metaclass for defining Abstract Base Classes (ABCs).
Use this metaclass to create an ABC."""
source: https://docs.python.org/3/library/abc.html
working since python2.6