remove interfaces.py (RFC)

[krenzlin]

interfaces.py declares interfaces (i.e. classes) which are meant to be used in the server part of pyoai.

There are some issues

1. All methods are missing the self argument
2. Interfaces in itself are not pythonic.
3. There are mixins in common.py which seem to serve the same idea.

While the interfaces are broken, they still have value as documentation and example for a server implementation.

My proposal is to move the comments from interfaces to the common mixins, maybe adding default implementations where its sensible and remove interfaces.py

[kmccurley]

I'm guessing that this project is abandoned.

[wetneb]

I am happy to merge and deploy sensible fixes @kmccurley (but if you do not depend on this library yet, please use something else, it has indeed many issues)