Relicensed with permission of David Fischer <djfische@gmail.com>.

* Provides the ability to pass around a single block of data instead
  of needing to handle transfering the nonce seperately.
* Opens up an avenue for automatic nonce handling in the future.
* Enables passing of nonces manually via explicit nonce parameters.

* Removes the broken ability to encode a nacl.public.Box
* Internal refactoring

* Uses lower level functions to generate a public_key from a private
  key.
* Reworks the generate method to just generate a random value.

The tests are adapted from the libsodium tests.
The docs are adapted from the rbnacl docs.