refs #1700 performance tracking in piwik.js

 * by default, send performance.timing.responseEnd - performance.timing.requestStart as generation time with the tracking request
 * if the performance.timing API is not supported by the browser, don't send the metric
 * new method disablePerformanceTracking()
 * new method setGenerationTimeMs(generationTime) to set the generation after measuring it on the server side