Implementation of HMAC-512-256 and HMAC-256

These are secret-key authenticators for data.  The provided
authenticator can be used to check for tampering, and timing independent
verification methods are provided.
Add utility functions for hex (en|de)coding
Provide utility methods for hex authenticators

These just hex encode or decode their inputs/outputs.
Factor apart keys into a file-per-class

This is mostly in anticipation of adding SigningKey/VerifyKey classes. I
feel some refactoring is in order here to keep the keys files/specs from
becoming overwhelming.
hex_* => hex*

For greater consistency with classes like Digest::SHA256.hexdigest.
Remove unneeded file
Move Authenticator specs to a subdirectory
Merge pull request #8 from namelessjon/dev/hmac

Implementation of HMAC-512-256 and HMAC-256