Add methods to allow deletion of axioms

[dosumis]

I'm particularly keen to be able to edit annotation property axioms using Brain. A method allowing deletion would be fine for this. A deleteAnnotation method could take the same form as the annotation method:

brain.deleteAnnotation("MyEntity", "MyAnnotationProperty", "MyValue");

When combined with a modified getAnnotation that returns lists (#10 (comment)), this feature would make it easy to use Brain for scripted editing of ontologies.