Add model.Factory to simplify Model instanciation

This class is meant to become the canonical way to instanciate model
objects.
Remove name-based Model constructors

In favor of model.Factory based approach.
Remove cpymad.service module

The CpymadService served both as factory and instance manager. The
latter should be the responsibility of user code, IMO, which is the
reason to replace this by model.Factory which has only one purpose.
Move all model constructor logic to Factory

In order to make all Model dependencies injectable, yet hide the concret
set of parameters required by the Model.__init__ function from the user,
it should never be called directly. The instanciation is now completely
controlled by the factory (which can more easily be customized than the
__init__ function, if desired).
Merge pull request #90 from coldfix/refine-model-creation

Replace CpymadService by a simpler model Factory