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.
Fix onDataSourceRemoved, and move onDataSourceChanged.
Viewer can now track which data source has the active clock.
Merge remote-tracking branch 'origin/master' into performanceNow
Update CHANGES.md.
Add more detail to CHANGES.md.
Use defaultValue.EMPTY_OBJECT instead of allocating.
Merge remote-tracking branch 'origin/master' into bingAttribution

Conflicts:
	CHANGES.md
	Source/Scene/CentralBody.js
	Source/Scene/CentralBodySurface.js
Fix error path when loading image via blob.

Returning an error from a error callback puts the code back on the success path, which is not what we want.
Make credits smaller and don't wrap them in the middle.
Merge pull request #1440 from AnalyticalGraphicsInc/tileLoadFixes

Fix a few minor issues in the tile load code
Re-add provider-level credits.
Factor out a getTimestamp function.