New way of handling global variables

Allows for overloaded function calls. Fixes #3

The code produces an error where g++ can't find func(int) even though candidate
func(...) is there... Behavior which is not present on my MinGW on windows.

Need to check if this is a bug, aka if clang & co. have it too.

To test, use `pythran -e test.py` and compile main.cpp. https://gist.github.com/coyotte508/7974708 and https://gist.github.com/coyotte508/7974719