Spring-based SPARQL built-in.

Dear Alex,

as announced in our mail conversation, here's the pull request for Spring+Sparql.

This patch introduces spring-beans and spring-context dependencies in mainline Prova. Besides, it creates a new maven profile called 'semanticweb', which if activated will pull in the OpenRDF dependencies and trigger the compilation of SemanticWeb related things. 
SPARQL built-ins live in ws.prova2.reference2.builtins.ProvaSparql*. Their only interference with the rest of Prova is in ProvaKnowledgeBaseImpl where they are registered as built-ins. This is where the Spring Framework will help us to avoid explicit dependencies on the SPARQL classes.

Hope this looks good to you
Best,
Malte