Implement NewKeyFromPassword

[gtank]

golang.org/x/crypto/scrypt is the right approach, but it introduces some critical questions:

1. N, r, and p need to be benchmarked on modern systems since the paper is from 2009.
2. The paper, Go docs, and broader internet lack clear guidance on generation & management of scrypt salts.

Until these are resolved, the package should continue to advise random AES keys.

[titanous]

This post and its follow-up have some useful recent benchmarks of KDFs: https://pthree.org/2016/06/28/lets-talk-password-hashing/

[jaylams]

Interesting benchmarks extensions