PBKDF2 support

There are already some other implementations
- http://anandam.name/pbkdf2/
- https://code.google.com/p/crypto-js/#PBKDF2
- https://gist.github.com/calvinmetcalf/91e8e84dc63c75f2aa53

PBKDF2 is important for Secure Remote Passwords (aka Client-side Proof of Secret).

I would be willing to help get this merged in.
