Develop

[mnmami]

Merge develop to master to bring the following features and improvements:

• Add support for subject filtering and subject projection
• Add support for single-star SPARQL queries
• Return schema and 20 results by default (to facilitate results parsing by e.g. Jetty)
• Fix group-by when other predicates are used than the one in aggregation function.
• Fix having to add the FILTER predicate to SELECT.
• Add exception handling and show meaningful error messages.