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

Conflicts:
	CHANGES.md
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.
Update CHANGES.md.
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
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