Improve documentation of new classes
Add test for Interface base class
Add cpymad.model_locator

This module is planned to provide adapters for use by cpymad.model that
separate the location/loading process of the model and its files from
the actual business logic.
Add tests for cpymad.model_locator.MergedModelLocator
Add test/**/__init__.py files

This enables the test directories to be treated as packages and the
tests to be loaded as modules from the root directory:

/path/to/pymad$ python2 -m unittest test.cpymadtest.test_model_locator