Add sun position automatic uniform.
Prototype data source plugins
Start adding sun rendering.
Ongoing data source prototyping.
Merge branch 'master' into sun

Conflicts:
	Source/Scene/SceneTransitioner.js
Frustum cull the sun.
Add fragment shader for pretty lame looking sun.
Update sun fragment shader.
Draw sun after the sky atmosphere.
Merge branch 'master' into sun
Merge branch 'master' into dataSources
Ongoing data source prototyping.
Merge branch 'master' into dataSources
temporary fix for #issue746.
Doc and clean up for new data source work.

Still needs specs.

I also reverted my changes to CesiumViewerWidget since I don't want to break the API just yet and it will be superceded by the "in progress" new Viewer widget.
When reverting the file, I forgot to maintain last new line.
Data source display cleanup.
Make CzmlDataSource asynchronous functions return promises.
Merge branch 'master' into dataSources
Merge remote-tracking branch 'origin/master' into issue746