Include the ability to sign data using ed25519

* nacl.signing.SigningKey represents a single signing key and
  gets intitalized with a seed value.
* nacl.signing.SigningKey.generate() creates a random seed
  and instiates a SigningKey with it.
* nacl.signing.SigningKey().sign() returns a bytes subclass that
  contains the signature and message concated together. However
  it also contains a .signature and .messge property to get the
  components seperately.