Improve server interface

Still work in progress...
Streamline database interface

The in-memory database backend now also uses ZODB, to provide a more consistent
interface. Furthermore, adding an object to the backend root automatically
commits a transaction to keep track of the object and assign it an ID.
Adapt clients update request method

It now uses the perspective broker of twisted.
Rewrite the server module

The server module should now work using twisted's Perspective Broker.