Get the value of a query string from a URL
/**
* Get the value of a query string from a URL
* @param {String} field The field to get the value of
* @param {String} url The URL to get the value from [optional]
* @return {String} The value
*/
var getQueryString = function ( field, url ) {
var href = url ? url : window.location.href;
var reg = new RegExp( '[?&]' + field + '=([^&#]*)', 'i' );
var string = reg.exec(href);
return string ? string[1] : null;
};