Support for "Stateless GetFeatureInfo"

This implements the "stateless GetFeatureInfo" approach as discussed in #2117.

With this patch `getGetFeatureInfoUrl` returns a URL even though there's no rendered layer for the WMS source. So it's now possible to display an OSM layer and use a non-rendered WMS source (with no corresponding layer) just for GetFeatureInfo.

Can be tested on http://erilem.net/ol3/wmsgfi/examples/getfeatureinfo-image.html and http://erilem.net/ol3/wmsgfi/examples/getfeatureinfo-tile.html. I'm particularly interested in tests in devices with pixel ratios greater than 1.

Closes #2117.

Please test and review.