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.
Fixes #473
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.
Hopefuly finally changes before pull request
Fixes #473
Merge pull request #562 from AnalyticalGraphicsInc/TileCoordinatesImageryProvider

Tile coordinates imagery provider
Merge remote-tracking branch 'origin/master' into labelOutlineWidth

Conflicts:
	CHANGES.md
Specs for outlineWidth

Also fleshed out other missing LabelCollection specs to bring coverage up to 100%.  Fixed a bug in `LabelCollection.remove`
Remove `clampToPixel` after review disussion.
Merge pull request #565 from AnalyticalGraphicsInc/labelOutlineWidth

Label clarity improvements