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

Conflicts:
	CHANGES.md
Small JulianDate cleanups.

* Use a scratch variable for leap second lookups instead of allocating.
* Add an instance compareTo method, use that from LeapSecond.compareLeapSecondDate
Fix a number of places that were using reference equality on JulianDate instead of equals.
Fix inconsistent name of cache variable
Merge pull request #558 from AnalyticalGraphicsInc/fixDateComparisons

Fix date comparisons to use equals
First cut at improved label outline support.
Add TileCoordinatesImageryProvider.
Use Cesium color instead of CSS color, and doc improvements.
More label improvements.
Add specs for TileCoordinatesImageryProvider.
Update CHANGES.md.
Share reproject types between layers but not contexts.
Avoid redundant ImageryLayerCollection events.
Fix doc typos.
Fixes #473
Update CHANGES.md.
Fixes #473
Cleanup outline code

1. Partially revert Sandcastle example
2. Update CHANGES
3. Fix a failing test (by removing an uneeded check).
4. Update simple.czml to take advantage of improved outlines.