PROV-835 Move password verification to LegacyAuthAdapter
PROV-835 Added PBKDF2 password hashing functions

Uses hash_pbkdf2 which is available in php 5.5+ and otherwise falls
back to custom implementation that requires the hash extension, which,
as far as I can tell, is part of most PHP setups (tested Debian, CentOS
and homebrew on Mac).
PROV-835 Implemented support for creating users via AuthenticationManager

Implementations for Legacy (md5-based) and CaUsers (pbkdf2 hashing) are
already working.