parse RST quoted literal blocks and class directives

A couple of additions to the RST reader, and associated tests, per discussion https://groups.google.com/forum/#!searchin/pandoc-discuss/class/pandoc-discuss/q1ELmf2et9o/MH75rcf_VrIJ.  

The note about how this is different from the rst2xml output should probably go in the docs somewhere, but I'm not sure where.