Make portable to both Python 2 and 3.
Rename rosteraccess to backend_roster.

This is the start of making selectable backends.
Use ConfigParser instead of python file for settings.

Make backend a class and allow importing of different backend modules.
Fix datastore pickling on Python 2.
Return empty set if error occurs.
Ignore .pyc files now that we're Python 2 compatible.
Add CSV backend and default to it.