Split into non-visual runner and visual reporter elements

[hastebrot]

Introduce <perf-runner> (non-visual) and <perf-reporter> (visual). The runner should report results of the on-going performance test via attributes and/or events to the reporter.

perf-tester/demo/runner.html

Lines 27 to 36 in 05908d5

	<perf-tester></perf-tester>
	<script>
	addEventListener('WebComponentsReady', function() {
	document.querySelector('perf-tester').tests = [
	'a.html',
	'input.html'
	];
	});

I wonder why the current <perf-tester> puts the results using string concatenation instead of templates into the text content. Is this to have a plain web-component (without Polymer's template stamps), or to not falsify the timing results?