Added HMACKeyVersion, HMACKeyVersionTest, SigningException, HashKeyVersion

Added:
HMACKeyVersion - class representing HMAC
HMACKeyVersionTest - JUnit tests for HMACKeyVersion
SigningException - custom signing exception
HashKeyVersion - abstract class extended by HMACKeyVersion
This is a HMAC-MD5, not SHA1. Also I don't think this comment is needed