raise RuntimeError for not allowed model properties

[csgui]

As stated by issue #198, a model property named properties is not correctly handled by couchrest_model.

In This line, properties from model is clashing with properties from the object itself. The fix is not straightforward and for now I'm proposing an exception guard.

This PR raises an RuntimeError in an attempt of use properties as a model property.

[aghyad]

Thank you @csgui for providing a visible exception.
+1