Add expiration support for message_verifier and message_encryptor

The patch will bake the expiration into message_verifier's signed message and message_encryptor's encrypted message to prevent the expiration from being forged. It will check the expiration every time you try to verify a signed message or decrypt an encrypted message. If the message is expired, it will raise an exception ExpiredMessage.
```ruby
encrypted_message = encryptor.encrypt_and_sign( data, 1.hour.from_now)
encryptor.decrypt_and_verify(encrypted_message) #=> data if not expired
encryptor.decrypt_and_verify(encrypted_message) #> raise ExpiredMessage if expired
```
cc @rafaelfranca @chancancode
Some quick friendly questions about the `value_array` name :smile:

Does it matter that the assigned variable is an `Array` or would a name like `values` be just as clear?
Can we make it clearer what the values signify and/or where they came from? What about something like `verifier_values` or `digest_values` for instance.