1. Added inspection of RDF materialization results to the Web interface.

* [DEV_SERVER,EXT] Added start_dev_server/1 which allows starting the development server on a non-default port.
* [DEV_SERVER,SC] The default port for the development server is now a setting.
* [RDF_MAT,DEB] Debug code is now only executed under the debug setting.
* [RDF_MAT,DEB] Store the most recent results for easy introspection (e.g., using RDF_WEB's rdf_mat_web/3).
* [RDF_WEB,NEW] Added rdf_explain_web/4 and rdf_mat_web/3, bringing easy introspection of materialization results to the Web interface.