Need ability to define custom verify password function for user login

We need to add encryption on user password. Currently Linz only allow ability to override the default  'verify password' function via using Linz.set() function, which I've tried and it is __not__ working. Linz did correctly set('verify password'), however the helpers-password is still loading the default verify password function.

I think it would be much cleaner to give user ability to define custom verify password function in the user model as shown in the first commit of this PR below (line 25 - 46).

If custom verify password function is not define if should either use the default one.