support Detector2 interface. refs #8

CFG based detector implements Detector2 interface. To test
these detectors, we have to support this interface.

This commit also contains another change: allow user setting original
IAnalysisEngineRegistrar instance. This is also needed to support
test for CFG based detectors.
Merge pull request #9 from eller86/support-detector2-interface

support Detector2 interface. refs #8