Renamed ifc.py to ifcopenshell.py and made it installable

Hi Thomas,
This commit is something I suggested some time ago, renaming ifc.py to ifcopenshell.py, and making it installable. It's just a suggestion, don't hesitate to close this request if you don't agree.

The idea is that I'm rewriting the IFC stuff in FreeCAD to use HEAVILY this new module, and making it installable will allow people to start using it :)

Cheers
Yorik