node client sdk: transaction security

[binhn]

We need ability to create secured transactions (sign with tcert) in the node.js client sdk. This is the code from fabric/core/crypto. We should refactor this module to provide functions that allow the application to create secured transactions prior to sending to the peer.

[smithbk]

This should be included in https://github.com/hyperledger/fabric/issues/1117

[smithbk]

@binhn Binh, I think this can be closed.