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
Changed to static final Strings and added support for more algorithms (MD5, SHA1, SHA256, SHA384, SHA512)