model-wire: Fixing compliance with CCNb format regarding Content tag

Previously, content didn't contain a proper BLOB tag

Refs #1020 (http://redmine.named-data.net/issues/1020)
model-wire: Adding partial compliance to CCNb format in Signature block

Now digest algorithm is specified as NOP and SignatureBits are padded to
16 bits

Refs #1021 (http://redmine.named-data.net/issues/1021)