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.
Merge pull request #661 from AnalyticalGraphicsInc/contributors

Add CONTRIBUTORS.md page listing all Cesium contributors.
Merge branch 'master' of git://github.com/AnalyticalGraphicsInc/cesium into czmlLayers