0.4.0

adds support for password-based key derivation (PBKDF2). this is a minor version bump because HMAC APIs have changed in a breaking manner.

instead of

let hmac:SHA256 = .hmac(message, key: key)

now do:

let hmac:SHA256 = .init(authenticating: message, key: key)

v0.4.0 also expands test coverage.