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.
Show view-specific Bing attribution.
Better mechanism for obtaining credits.
Handle the possibility that no attribution is provided.
Add getTileCredits to all imagery providers.
Update tests with updated URL.
Remove useless comment.
Move terrain/imagery credit update to surface.
Start of model tests
Add renderForSpecs
Added new model for testing
Picking and other new tests
Removed DeveloperError doc for required parameters
More tests
Zoom tweak
Start of animation and skinning tests
More animation tests
Merge remote-tracking branch 'origin/master' into performanceNow
Update CHANGES.md.
Add more detail to CHANGES.md.