create supplement.Rmd
- moves implementation details and S4 subsetting to supplement
- adds a render call to the Makefile to make supplement pdf
- removes XPath query on RDF
Merge branch 'devel' of github.com:ropensci/RNeXML into devel
get_rdf() now uses local XSLT instead of REST API
This commit updates travis and documentation regarding the Sxslt dependency
Also updates the rdf unit test (by removing the XPath query and by checking document type)
Addresses #73.

Should probably update sparql.R example to actually use this new get_rdf() command
instead of starting with the already extracted RDF