Some rework on wmts, introducing the wms filter plugin

* wmts: There is currently a bug in the GeoScript gem, which gives
unreliable results when projecting coordinates from lv03 to wgs84
(see https://github.com/scooterw/geoscript-ruby/issues/4).

* renaming wxs to wmts: I think it would be better if the different
filters for the different protocols have their own ruby code file.

* wms: Introducing a new filter for WMS queries.