Control cached cart widget based on detecting cache enabled from WordPress settings 

* Detect cache enabled from WordPress settings.
* Allow cache config to be set in WP_CONFIG.PHP
* Cart Widget will now respect cache detection setting.
* Check using $cache_enabled setting removed because that setting can't be found in WordPress documentation.
* WP_CACHE defined constant user per documentation here http://codex.wordpress.org/Editing_wp-config.php#Cache

Also,
* Add ability to define some constants in WP_CONFIG if desired