Allow to POST the url of a WFS service or layer (#392).

Use "owstype":"WFS" in the POST'ed JSON to use this.

- Rename some functions from 'WMS' to 'WxS'
- Pass service/layer type around
- force srsName on the WFS protocol for a proper projection (doesnt check
if the layer actually supports the given projection though..)

Tested against GeoServer 2.3 (both loading WFS layer and/or service),
known to not really work when trying to load a WFS layer from old
MapServer instances (ie WFS 1.0..)