Add an Encoding system instead of adhoc uses of binascii

* 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.

Update libsodium to 0.3

Fixed ignore file

apostrophe & capitalization in docs/signing.rst

Digital signatures

* Accepts the binary representation of a public key
* nacl.signing.VerifyKey().verify accepts either a single signed
  message with the signature and message conatted or seperate
  message and signature and will raise an exception if it detects
  tampering, forgery, or corruption.

* 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.

Remove support for libnacl leaving only libsodium

The reference implementation of NaCl hasn't seen a release in a
long time and libsodium has several additions we require.

Properly handle strings with internal nul

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.

Add nacl.random(N) to generate a random N bytes