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