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.
Should be WMTS not WMST.  Also capital P in ImageryProvider, and `WMTSImageryProvider` should be updated to the new name.