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.
Merge pull request #2838 from mike-mcgann/wmts-tile-class

ol.source.WMTS now passes tileClass option to ol.source.TileImage
Constrain center in zoomify example
Merge pull request #2847 from elemoine/zoomify-example

Constrain center in zoomify example
Issue #2802: Update OSM attribution
One OSM attribution
Merge pull request #2857 from tschaub/osm-attribution

Update the OSM attribution.
Document exportable vector source symbols
Merge pull request #2859 from tschaub/vector-doc

Document exportable vector source symbols.