Fix Billboard.computeScreenSpacePosition.
Update CHANGES.md.
Merge branch 'master' into computeScreenSpacePosition

Conflicts:
	CHANGES.md
support for for reading dynamic ellipse information through czml.
fix formatting in CZMLDefaults.js.
Update CHANGES.md.
changes based on czml-writer updates.
add tests for dynamicPolyline and dynamicPolygon.
clean up code, improve performance, increase code coverage for dynamicEllipse.
Merge branch 'master' into computeScreenSpacePosition

Conflicts:
	CHANGES.md
	Source/Scene/Billboard.js
	Source/Scene/Label.js
Fix after merge.
Update based on review.
Revert BillboardCollection and update CHANGES.md.
Fix memory leak in Material.
Merge pull request #567 from AnalyticalGraphicsInc/destroyMaterial

Destroy sub-materials.
Add comment to computeScreenSpacePosition
Merge pull request #557 from AnalyticalGraphicsInc/computeScreenSpacePosition

Fix Billboard.computeScreenSpacePosition.
Merge branch 'master' into dynamicEllipse
Build speed improvements.

Refactor build tasks to use shared functionality for iterating files matched by filesets, and for writing content to files.  Use a state file to track whether comments have been removed from shader source.
Make sure the build directory exists before running any of the tasks.