Add support for PBKDF2 to PasswordHash

Add support for PBKDF2 to PasswordHash

Integrate the work of pingunaut to add support for PBKDF2 password
hashing. A new class `PasswordHashPbkdf2` is added, which builds
on his `SecurePasswordHashUtils` class, but makes it a subclass
of `PasswordHash`. This will replace the original class when
integrating the new PasswordHash way into GitBlit.