Create mapper object, instead of monkey-patching the model class

[lowinger42]

Pros:
A model instance is not tied to a specific sql driver, so working with for example a local db and a remote over json gets easier

Example:
customer = remotedb.load(Customer(22))
localdb.store(customer[0])

This is also the reason the model object should not have .load/.store methods, it would lock the instance to a specific database driver.

The mapper can be used in custom code, for example in the wsgi api.py