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.
I guess it's easiest to change to `projection: featureType.srs` and leave it to OpenLayers to instanciate the projection object then. That way, projection is null as it has been if `featureType.srs` is undefined. You're okay with that?