Rehash branch

As requested, a pull request for a legacy hash rehasher. However, having finished this, I'm not sure it really adds much--the actual functional part is very simple. The original intent was to hide all knowledge of the php password_hash() and password_verify() functions from the user, but I realized that the implementation of the testing for legacy hashes still requires the user to use password_verify in the validationCallback, so that benefit isn't really achieved. I would completely understand if you decide that this isn't worth maintaining, and decline the pull request.