Don't inherit settings.base when extending a model

Fix the bug in `ModelClass.extend` where the `base` option used
in the new class was inherited from ModelClass. As a result
the extended model was incorrectly based on ModelClass's parent.

Modify `modelBuilder.define` to normalize the property name storing
the name of the base model to `settings.base`.
Merge pull request #323 from strongloop/feature/do-not-inherit-base-model-option

Don't inherit settings.base when extending a model