Add Styled Layer Descriptor (SLD) version 1.0.0 parser (read/write)

Let me first warn this is not the full Monty, it's partial and mostly tailored at what we currently support in ol3 wrt styling. So in order to create a fully fledged style editor round-tripping SLD from the server more work will be needed on this in the future, but IMHO it's a decent enough start.

TIA for any review.