Change WPeC customer cookie performance and synchronicity

* Wrap the check for the WPeC customer cookie in a function
* Rather than check for the customer cookie on the page load set a timer that should cause the cookie check to happen very shortly after the ready event processing completes
* Fix parsing of the customer cookie check response to retrieve the customer id from the AJAX response generated with the PHP function wp_send_json_success
* When the cookie is present it is parsed to extract the WPeC customer id, this ensures that the customer ID is set when the cookie is present just the same as when the cookie is being created
* Added some console messages to aid WPeC customer support in tracking down performance and reliability issues that have been reported in the WordPress support forum during the summer/fall of 2014
* Added timeout value and a comment in the source that be enabled if a site wants to allow processing to move forward without setting the customer id in the javascript

Fix for issue #1597