* Fixed compilation with OpenSSL 1.1 and PHP 7.1
* Added KDF abstract class to be a parent for all key derivation function classes
* Added PBKDF2 class extending KDF class and implementing PBKDF2
* Renamed HashException code ALGORITHM_NOT_FOUND to HASH_ALGORITHM_NOT_FOUND
* Renamed MACException code ALGORITHM_NOT_FOUND to MAC_ALGORITHM_NOT_FOUND