Added the ControlParameters class.
ForwardingFlags: Added getNfdForwardingFlags and setNfdForwardingFlags.
security: Fix bug in the IndentityManager constructor to create the default FilePrivateKeyStorage.
security: Fix bug in KeyChain.sign: Need to pass the target to sign the array.
util: Added NfdCommandInterestGenerator.
encoding: Implement encodeControlParameters.
security: In FilePrivateKeyStorage, implement sign.
Node: Added _nfdRegisterPrefix.
Face: In registerPrefix, if setCommandSigningInfo was called, send an NFD registration request.
tests: Added test_publish_async_nfd.