Set projection on WFS-layer retrieved by capabilities request.

The change makes it easy to add a WFS-layer retrieved via a capabilities request. 
If the layers projection differs from the map projection, features are now projected accordingly (requiring proj4js).

If you consider this worthwhile I'm happy to send a CA.
looking at the OpenLayers code for WFS 1.0.0, there is at least a theoretic chance that featureType.srs will not be set, so we can add a check here?