Added more tests for NFNMaster
ContentStore is no longer a singleton but an instance in the NFNMaster; added more NFNMaster tests
NFNMaster spec runs both local and network implementations, some tests fail for network code
Fixed bug: local abstract machine instantiated call executor with itself instead of nfnMaster
Created a singleton object to start a native ccn-lite nfn engine. This enables unit tests for the full stack. However, since the nfn engine is strictly synchronous, asynchronous communication tests fail and the test also fail if executed as a suite, because the next test can only succeed if the previous test is done.