TweetNaCl is now a Module instead of a class

It seems cleaner to define TweetNaCl as a module rather that a class as
most of the API wil then be defined within this module.
Add an empty TweetNaCl module for further use
Implement crypto_secretbox

Changes:
========

* Added crypto_secretbox
* Added crypto_secretbox_open
List all the function that will be supported
Add method aliases
Ensure all the test files are executed
Use assert_raises instead of custom exception catching
Add KeyPair and CryptoBox objects