Fix label removal bug

1. See https://groups.google.com/forum/?fromgroups=#!topic/cesium-dev/pqS_8Z5Tb1g for the initial bug report.
2. Removing a label resulted in `label._index` being incorrect, which ultimately resulted in destroyed labels remaining in the collection and the incorrect label being removed.
3. Added unit test to deomnstrate the problem.
Update CHANGES.
Merge pull request #452 from AnalyticalGraphicsInc/labelRemoveBug

Fix label removal bug
Fix conic visualizer sensors

Fix incorrectly cached directions in the cone visualizer which was causing all conic sensors to have the same geometry.

This fixes #242
Merge pull request #455 from AnalyticalGraphicsInc/coneFixes

Fix conic visualizer sensors
Merge branch 'master' into background

Conflicts:
	CHANGES.md