Added custom validity checking delegate method to PAPasscode

So the general idea behind PAPasscodeViewController:checkPasscodeValidityWithEntry: is to enable the developer to implement a custom validity test for the entered passcode using whatever arbitrary means they see fit.

The reason why we implemented this delegate method is because for our solution, we store the passcode as a one-way hash that we obviously cannot unhash. Using this delegate method, we can hash the candidate entry and see if the hashes match.

Hope this helps!