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 demonstrate the problem and removed the _index properly completely in order to fix it.