Update Cesium to use new QuantizedMesh extension format.  In place of using a bitfield identifying the presence of an extension, all extensions must start with a unique id followed by a 32 bit length of the extension, followed by the extension bytes.  This new format allows for clients to skip over extensions that it does not know of or care to unpack.
Updated unit test datafile defining a terrain tile that includes the oct-encoded extension data using the revised extension format.
Merge remote-tracking branch 'agi/master' into quantizedMeshExtensions
Added two new unit tests to test QuantizedMesh tiles with unknown extensions defined.  The unknown extensionId is 200, which has not been assigned to any extension.
Merge pull request #1993 from abwood/quantizedMeshExtensions

Quantized mesh extensions