Included 4Store binding into ocaml-rdf

I only put my files into src/lwt. I don't know how to include them in the compilation, moreover with lwt dependencies.

It may be better to create another module for the lwt lib, and later the async lib in order to well differentiated them. Which will do this: Rdf_lwt.Sparql_http and Rdf_async.Sparql_http.
In that idea, we need to find another relevant name for Rdf_lwt.4store which is not allowed in ocaml.