util: In SignedBlob, keep an internal signedBuffer.
uti: In Blob added isNull().
Signature: fix bug in setSignature().
Data (internal): Removed startSIG, endSIG and rawSignatureData. Instead wireEncode and wireDecode set wireEncoding as a SignedBlob.  Make sign and verify use wireEncoding.
wscript: Move the include for securityLib files near the top of ndn-js.