add support for WebFeatureService

[raphaelquast]

Add support to draw geometries obtained from WebFeatureServices

• use cartopy.feature.WFSFeature

I don't have any experience with WFS yet but it would be nice to have...

• collect a list of open-access WFS services
• add a m.add_wfs function to draw WFS service layers
• add dot-access functionality of pre-defined services (similar to m.add_wms)
• [maybe] add direct support to make geometries pickable?