WMTS Support addition

Contribution for WMTS 1.0.0 support in cesium JS.

The spec.js file (Specs/Scene/WMTSImageryProviderSpec.js ) still needs to be substantially filled.
Some tutorials or examples of use should also be added.
These values are specified in the Capabilities document, right?  Unless you feel that 256x256 is so common that no one is likely to ever use anything else, we should at least allow the width/height to be specified to the constructor.  Ideally, we'd get it from the capabilities document automatically.