ol.source.WMTS now passes tileClass option to ol.source.TileImage

The constructor for `ol.source.TileImage` accepts an option for the `tileClass`, but `ol.source.WMTS` does not pass this to the parent. This pull request adds that in. 
While you're at could you please add more docs for `tileClass`, both for `WMTSOptions` and `TileImageOptions`? Thanks.