WMTS createLayer method improved

This PR adds several improvements to the WMTS createLayer method:
 - one can now specify which projection is desired for the created layer. The first Tile Matrix Set whose supportedCRS property matches the given one is chosen.
 - if tileMatrixSetLimits are set in the tileMatrixSetLink, the resolutions, server resolutions, minScale and maxScale properties should now reflect those limits.
 - layers created from geoserver WMTS capabilities do not always have a style (according to the spec, this looks like a geoserver bug). In this case, an empty style identifier is used.