Add support for official W3C SPARQL compliance tests

[Callidon]

Here we are: we now have all the features required to match SPARQL 1.1 compliance. However, we need to run the official benchmark for W3C SPARQL compliance on the framework in order to claim such compliance.

To do so, we need a few things:

• A runner that can automatically load W3C SPARQL compliance tests and execute them using sparql-engine.
• A method for comparing sparql-engine query results with the expected test results.

For me, the main issue is that the W3C SPARQL compliance tests rely on XML output files to compare the expected output of query execution, and the syntax can be very permissive.