pageview handler
@method pageview
@param {Object} payload payload object
@param {Object} payload.tracker page title
@param {Object} payload.title page title
@param {Object} payload.location page location
@param {Object} payload.url current url
@param {Function} calback callback function
function (payload, callback) {
                var args = {},
                    argsKeys = ['location', 'page', 'title'];

                argsKeys.forEach(function consumer(prop) {
                    if (payload.hasOwnProperty(prop)) {
                        args[prop] = payload[prop];
                    }
                });

                args.hitCallback = callback;
                // `page` is alias to `url`
                if (payload.hasOwnProperty('url')) {
                    args.page = payload.url
                }

                _command.call(this, {
                    tracker: payload.tracker || '',
                    commandName: 'send',
                    arguments: [
                        'pageview',
                        args
                    ]
                });
            }