Added skyBoxSources option
Merge remote-tracking branch 'origin/master' into skybox
Improved skybox after review
Updated doc
Merge remote-tracking branch 'origin/master' into skybox
Merge master to skybox
Merge master to skybox
Merge master to skybox
Merge remote-tracking branch 'origin/master' into skybox
Merge remote-tracking branch 'origin/master' into skybox
Merge remote-tracking branch 'origin/master' into skybox
Changes based on review
Merge master to skybox
Restore prototype equals functions to core types.

I also added data comparison options to `TimeInterval` and `TimeIntervalCollection`.

All of this is needed for `DynamicScene` Geometry & Appearances support, but I figured I'd bring it in under a separate pull to keep things manageable.
Changes after review.
Merge remote-tracking branch 'origin/master' into prototype-equals

Conflicts:
	CHANGES.md
Merge pull request #1231 from AnalyticalGraphicsInc/prototype-equals

Restore prototype equals functions to core types.
Merge remote-tracking branch 'origin/master' into skybox

Conflicts:
	CHANGES.md
Merge pull request #1196 from AnalyticalGraphicsInc/skybox

Added skyBoxSources to the CesiumWidget and Viewer constructors
Trim trailing whitespace.