Password hashing

[drmathias]

Based on the proposed ticketbooth-solutions/Ticketbooth/issues/24, it would be useful to implement a wrapper for a password hashing algorithm which is best suited for smart contracts. Possibilities include Argon2di, PBKDF2, scrypt or bCrypt.

The most important factors I would consider for choosing an implementation are hash size and security. The smaller the hash size, the less gas which is required to log it in a contract, though a hash that is too short will have many more collisions. Splitting the hash from the salt would be benefitial for gas saving, as the salt does not have to be stored in a smart contract to verify a hash.