SPARQL query language support

[georgiy-belyanin]

It would be nice to support SPARQL as one of the query languages.

Here is an example query that matches book title.

SELECT ?title
WHERE {
  <book1> <title> ?title .
}

Let's omit prefixes and assume we currently support only SELECT ... WHERE ... queries for now.
Some related implementation are available in la-n-egg-rpq.