* 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.

The strings from NaCl can contain nul characters inside the string
and are not nul terminated. Using ffi.buffer with an explict
size will prevent issues here.

* Supports sha256 and sha512