First phpBb users import + successful authentication of a phpBb user

* the users authentication is no more hard-coded in "actions/sign-ini-target.php", but handled with "auth.user.check-signin-credentials" Plugins Hooks
* the "auth" Plugin implements that Hook, and checks that the DB User provided is "talk-talk"
* ArrayUtils moved from "CorePlugins\Utils" to "Core\Utils"
* PluginManager BaseBehaviour is now abstract, and has a direct access to the App
* the Plugins "enabledOnlyForUrl" can now be an Array
* some bugfixes with Illuminate Database multiple connections management
* first successful phpBb 3 users import (just a simplistic one, for the moment)
* first successful phpBb 3 authentication, thanks to  "seyon/phpbb3-user-bundle"
* added Underscore.php library
* added PHPBB3UserBundle library
* misc tiny improvements & refactoring