Port of the OpenBSD `bcrypt_pbkdf` function to pure Javascript. `npm`-ified
version of [Devi Mandiri's port]
(https://github.com/devi/tmp/blob/master/js/bcrypt_pbkdf.js),
with some minor performance improvements. The code is copied verbatim (and
un-styled) from Devi's work.

This product includes software developed by Niels Provos.

## API

### `bcrypt_pbkdf.pbkdf(pass, passlen, salt, saltlen, key, keylen, rounds)`

Derive a cryptographic key of arbitrary length from a given password and salt,
using the OpenBSD `bcrypt_pbkdf` function. This is a combination of Blowfish and
SHA-512.

See [this article](http://www.tedunangst.com/flak/post/bcrypt-pbkdf) for
further information.

Parameters:

 * `pass`, a Uint8Array of length `passlen`
 * `passlen`, an integer Number
 * `salt`, a Uint8Array of length `saltlen`
 * `saltlen`, an integer Number
 * `key`, a Uint8Array of length `keylen`, will be filled with output
 * `keylen`, an integer Number
 * `rounds`, an integer Number, number of rounds of the PBKDF to run

### `bcrypt_pbkdf.hash(sha2pass, sha2salt, out)`

Calculate a Blowfish hash, given SHA2-512 output of a password and salt. Used as
part of the inner round function in the PBKDF.

Parameters:

 * `sha2pass`, a Uint8Array of length 64
 * `sha2salt`, a Uint8Array of length 64
 * `out`, a Uint8Array of length 32, will be filled with output