Allow users to specify a callback to enforce a password policy
Wrapped password policy callback in list to prevent it from being bound
changed password_policy_callback to user_policy_callback

user_policy_callback takes all json data as an arg, and must return it (potentially modified) or raise an exception
added self.user to user_policy_callback args
Merge pull request #14 from dankrause/issue_10

Allow users to specify a callback to enforce a password policy