Update GeoExt.data.PrintProvider for basic XYZ layer format support

It's just similar encoder like that OSM one, but with extension autodetection and tileOriginCorner coming from OpenLayers. I'm not sure, if there's need to implement path_format and how should look customParams (as string like ?param1=value1&param2=value2, or Ext.encode[{param1:value1,param2:value2}]).