Backport to Python 2.7

Unfortunately, twisted is not yet fully ported to Python 3.x. Therefore, we
have to change our codebase back to comply with Python 2.7.
Revise client interface for the loader
Fix ServerModelLoader

It is now adapted to use twisted.
Correct type in user documentation
Separate server functionality from database

The database and user classes are a logically separate part of the library.
Change interface to use twisted pb

Twisted's Perspective Broker provides the optimal tool for creating an entirely
customized protocol. This is work in progress.
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.
Rename factory classmethod

`fromFile` seems more technology-agnostic than `fromZODBFile`.
Fix client bugs and handle errors

There were some bugs in the untested (o.O) high-level client logic, which are
now fixed. Furthermore, the client application can now handle connection
errors.
Use logging for error message

Always beats print statements.
Add an example of black triangle

The stub example covers server-client interaction without actually doing
anything with the established connection. This merely demonstrates the very
basic functionality of setting up a server and a client that communicate with
each other.
Merge branch 'twisted'

The branch is now reasonably stable, since the basic server-client example
works.