RDF reading writing

##Description

RDF/RDFA is a common spec for semantic data storage. R /used/ to have a library, RDF, for handling it - but it was removed as it became incompatible with R version increases.

It would be good to have a replacement!

Note that ropensci has an RDF implementation under way by wrapping the Redland RDF C library, see ropensci/redland-bindings. Comments for requested features / functionality would be appreciated there.

##Necessary features

1. Read an RDF/RDFa set into an R object
2. Turn an R object into RDFa

##Endorsements and additional suggestions

1. Oliver wrote this and endorses all of it.
2. Carl thinks this is a good idea and added the links to the redland-bindings repo

##Volunteers and work