Merge from vendor branch TNF:

Merge from vendor branch TNF:
Importing security/ruby-hmac 0.4.0.

This module provides common interface to HMAC functionality. HMAC is a
kind of "Message Authentication Code" (MAC) algorithm whose standard
is documented in RFC2104. Namely, a MAC provides a way to check the
integrity of information transmitted over or stored in an unreliable
medium, based on a secret key.

Originally written by Daiki Ueno. Converted to a RubyGem by Geoffrey Grosenbach