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.
Need to briefly mention what's supported (and maybe what's not) by this imagery provider, since it doesn't support the entire 1.0.0 spec, right?