WMTSCapabilities: support of TileMatrixSetLimits, LegendURL and InfoFormat

This PR:
 * adds support for TileMatrixSetLimits and InfoFormat, 
 * improves support of LegendURL: there can be more than one legendURL per style. The WMTS 1.0.0 spec indeed says that "A WMTS ServiceMetadata document may include zero or more LegendURL elements to provide an image(s) of a legend relevant to each style of a layer." (OGC 07-057r7, p21)