Revised implementation of GA event tracking after feedback on #1315.
Adds jQuery .click bind on add to cart buttons on both single product
pages and loop pages.
new filter:
woocommerce_ga_event_tracking_parameters - allows change of category,
action, or label parameters included in _trackEvent
closes#1256
Warning: call_user_func_array() expects parameter 1 to be a valid
callback, class 'WC_Google_Analytics' does not have a method
'google_tracking_code' in /htdocs/wp-includes/plugin.php on line 405
tracking functions were inside of init_form_fields()