Update for a partial management of the max size of uploaded files.

- Added : marker of end of conf as a comment in installer/default/configuration.php in order to permit addition of new parameters,
- Updated : the add method in the Config class in installer/models/config.php in order to permit addition of new parameters,
- Added : setup of the max size for file upload in the installation process in installer/index.php,
- Added : support of the uploadmaxsize parameter in component/com_stream/controllers/system.php,
- Updated : controller, view, template of the advanced settings configuration page with support of modification of the uploadmaxsize parameter.