[Rdfapi-php-interest] New ModelFactory methods

Hi all,

There are two new methods in ModelFactory:
     getOntModelForBaseModel($baseModel, $vocabulary)
     getResModelForBaseModel($baseModel)

They create ResModels and OntModels from an already existing model.

The first one was necessary to make Model::getOntModel work. For  
consistency, I also added the second one.

There are a bunch of new unit tests for this in /test/unit/Model/ 
Model_tests.php and /test/unit/Model/ModelFactory_tests.php.

Best,
Richard