cryptography update with cleaner module

[cryptohazard]

This code implements the following features:

• create a crypto module to regroup all the functions
• uses steem-js crypto as subfolder
• stop sending user private key to server
• instead create a shared secret from user memo and server memo keys
• add a fix hmac derivation (this can be easily extend to have session keys)
• note: users can authenticate the server (Lara) the same way they are authenticated by the server (not implemented yet)
• prepare switch from memo encryption to AES-GCM encryption (this will improve the overall speed)

TODO

• clean all the places we send the private key to the server
• clean up the logs
• move the key storage from cookie to local/session storage
• add key storage encryption
• use Ephemeral DH
• use one crypto library instead of many

[kingswisdom]

Thanks for your contribution !