node/tests: In test-encode-decode-data dumpData, check if the signature is Sha256WithRsaSignature.
Signature: In the default constructor, set the signature to null, not Buffer(0);
security: In Identitymanager.signByCertificate, iniitally set an empty signature so that we can encode.
node/unit-tests: In test-data-methods, port all tests from PyNDN.
util: Added ChangeCounter.
Name, MetaInfo, Exclude, PublisherPublicKeyDigest: Added support for getChangeCount.
KeyLocator (doc): In setKeyData, the param is Blob, not Buffer.
 encoding: Fix bug in Tlv0_1WireFormat: Need to require PublisherPublicKeyDigest.
node/unit-tests: Ported test-interest-methods from PyNDN.