* This eliminates the global state from the encoding system.

* Adds an Encoding system that supports raw, hex, base16, base32,
  and base 64.
* Adds an encoding=ENCODING parameter to methods to control what
  encoding data is returned/passed in.
* encoding parmater supports passing in a custom encoder object
* Encoding system supports registering of new encoders through
  nacl.encoder.register.

* 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.

* Supports sha256 and sha512