Add autogeneration of RSA key pair for each shuffling session

[Velnbur]

Situation

Currently, for using node-cli, the user must somehow generate an RSA key pair before starting shuffling (usually, with openssl binary or even an online generator) which makes the shuffling process really uncomfortable and bloated amount of parameters that the user should pass to node-cli

Objective

Add a feature for the automatic generation of RSA key pair for each Coin Shuffle session

Acceptance

When a user doesn't pass a his RSA secret key, node-cli generates a new one on fly