enable use of derived keys

Scrypt is a key derivation function. Although its use for pure password hashing and verifying seems more prominent, it is very suitable if you just need that derived key. Unfortunately this node.js module does not provide any access to the derived key.

This PR changes that by exposing the actual key in passwordHash and verifyHash functions.

Using the passwordHash and verifyHash functions for key derivation is usefull if you do not want to use this module for encryption and decryption but still need a passphrase-derived key for other purposes.