Fixed the argument errors for Authenticators to maintain constistency between classes
Merge pull request #18 from razorinc/issue_17_authenticators

Consistency between Authenticators and Signing Keys

Fix authenticator part of Issue #17
SecretBox usage instructions

Show the Crypto::SecretBox API:
* how to generate a key
* a simple nonce strategy
* encrypting a message
* decrypting a message
* decrypting a corrupted message
Finish basic documentation for SigningKey
Crypto::VerifyKey documentation
Ed25519 documentation in README
Missed an italic
HexEncoder example
These aren't all necessarily 'security' features
Ed25519 diagram
Fix up YARD docs for VerifyKey

This also closes #16