Updated documentation

phpGrabComics documentation has been out-of-date for much time, and it was not covering many of the features of the software.
The new phpGrabComics HOW-TO (for users of phpGrabComics) is available at http://www.baravalle.it/phpGrabComics/how_to.php
The new phpGrabComics guide for modules developers is available at http://www.baravalle.it/phpGrabComics/modules_development.php