ReferenceProperty availability

Make ReferenceProperty have the same availability as the DyanmicObject that the reference is on.
Merge pull request #657 from AnalyticalGraphicsInc/referenceAvailability

ReferenceProperty availability
Add CONTRIBUTORS.md page listing all Cesium contributors.
Overlap frustums.
Merge pull request #661 from AnalyticalGraphicsInc/contributors

Add CONTRIBUTORS.md page listing all Cesium contributors.
Merge remote-tracking branch 'origin/master' into overlapFrustums
Merge pull request #663 from AnalyticalGraphicsInc/overlapFrustums

Overlap Frustums