func NewScrypt(N, r, p int) (func(salt, password []byte) []byte, error)
NewScrypt returns a new key derivation function that uses scrypt to do
the derivation. The returned key will be 32 bytes in size.
If N, r, or p are invalid nil and an error are returned.
See code.google.com/p/go.crypto/scrypt#Key for details on proper values for
N, r, and p.