Update interfaces.py

Probably a copy paste error from the docstring above
Merge pull request #190 from maikroeder/patch-3

Update interfaces.py
Actually declare the API of the sdiapi object.
Tidy up interfaces, and point to the packages that use them.
Fix coverage under Py3k.
Switch 'is_service' to use IService marker interface.

Adjust tests, etc. accordingly.

Add an evolve step to update services in the root.
Add a 'Services' folder contents tab.

Segregates IServices from normal content.
Redirect to '@@services' after adding service objects.
use alsoProvides instead of directlyProvides in case obj already has instance-level interfaces
use alsoProvides instead of directlyProvides in case obj already has instance-level interfaces
make work on py2