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.
You're right : these two parameters should be modified by users at construction.

> Ideally, we'd get it from the capabilities document automatically.

To me WMTSImageryProvider just has to compose GetTile request with parameters that has been given at construction. GetCapabilities request handling (if needed) should be treated by an other component.