encoding: Added WireFormat. decodeSignatureInfoAndValue.
security: In PolicyManager make skipVerifyAndTrust, requireVerify and checkVerificationPolicy take an Interest as well as Data.
security: In SelfVerifyPolicyManager make checkVerificationPolicy also take an Interest and use wireFormat.decodeSignatureInfoAndValue to parse the command interest.
security: In KeyChain, added verifyInterest.
tests: Make test_encode_decode_interest sign and verify the freshInterest to test signed interest.