Add Configuration.get_instance to ease handling of singletons
Clean up test_config a bit, avoid duplication
Avoiding usage of weakref

Reasoning is that we could end up with using the instance ocassionally and then
have each time a fresh instance if we do not keep a reference. For that reason
I take a dict here for now.
Add local import to avoid hick up in Travis runs
Merge pull request #10 from johbo/gup-114-config-get-instance

Add Configuration.get_instance to ease handling of singletons
Use system Cython packages
This will probably fail
Remove it also from setup.py