CRM_Utils_System_WordPress - Fix is_wordpress

The property is defined in the base-class but never set.
api/explorer - Add support for andyw's wp-cli integration

(See also: https://github.com/civicrm/civicrm-wordpress/pull/55 )
Merge pull request #4126 from totten/4.5-wpcli-explr

api/explorer - Add support for andyw's wp-cli integration