Use performance.now to limit load queue processing time.

Previously we were using Date.now, but its resolution is terrible, at
least on Windows.  Cesium is only supposed to spend up to 5ms per frame
processing the tile load queue, but WTF reported it frequently taking over
10ms.  After this change, processing time is actually limited to ~5 ms,
which reduces frame rate drops while loading tiles.
Merge remote-tracking branch 'origin/master' into performanceNow
Update CHANGES.md.
Add more detail to CHANGES.md.
Factor out a getTimestamp function.
Use getTimestamp in PerformanceDisplay too.
Merge remote-tracking branch 'origin/master' into performanceNow

Conflicts:
	CHANGES.md
Use getTimestamp in Clock.
Merge pull request #1439 from AnalyticalGraphicsInc/performanceNow

Use performance.now to limit load queue processing time.