createEngine() and createEngineDescription() factory methods

Instead of the createAggregate(), createPrimitive(), createAggregateDescription() and createPrimitiveDescription() methods, there should be more generic methods:

createEngine() - should cover all current createAggregate(), createPrimitive() methods and also allow to create an aggregate AE from a variadic list of descriptors
createEngineDescription() - should cover all createAggregateDescription(), createPrimitiveDescription() methods

See also: https://code.google.com/p/uimafit/issues/detail?id=105

Instead of removing the existing methods, they should only be deprecated for now to ease the transition.