1533 - Add support for Element Timing

[tkadlec]

Nice and shiny Element Timing API support thanks to Chrome's new trace events.

This PR closes #1533 by adding Element Timing to:

The JSON results:

"elementTiming": [
    {
        "identifier": "post-title",
        "time": 2070,
        "elementType": "text-paint",
        "url": ""
    },
    {
        "identifier": "post-hero",
        "time": 2200,
        "elementType": "image-paint",
        "url": "https:\/\/res.cloudinary.com\/webpagetest\/image\/upload\/f_auto,q_auto,c_fill,w_640,h_512\/august-roundup_plhjt4.png"
    }
],
"elementTiming.post-title": 2070,
"elementTiming.post-hero": 2200,

The custom waterfall page (using the same lines as the user timing API):

The plot full results page:

And the HTML table on results pages:

[scottjehl]

This looks really nice. Useful addition!