[feature/auth refactor] Refactoring of auth_plugins into classes and provides basic unit tests

This refactors all of the current auth plugins (db, apache, and LDAP) to be new authentication provider services. It copies verbatim the internals of each plugin's function into the methods provided by each new authentication provider class. It removes globals from each method and replaces them with class variables passed to the provider during construction. It provides basic unit tests for the apache and db providers.

Unit tests can not be added for the ldap provider without additional refactoring of the ldap provider so as to remove direct calls to PHP's ldap functions so that a mock LDAP provider could be used.

This is part of the GSoC project detailed in https://www.google-melange.com/gsoc/proposal/review/google/gsoc2013/hardolaf/13001

http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=43739
http://tracker.phpbb.com/browse/PHPBB3-9734