An implementation of the SPARQL 1.1 Graph Store Protocol

This pull request is intended to get feedback. There are some issues to be addressed before merging:
- Is it correct to use `Dataset` in this setting?
- The issues RDFLib/rdflib#303, RDFLib/rdflib#302, and RDFLib/rdflib#301 need to be solved before the code of this pull request works. (I worked around this by modifying rdflib a bit, but it is not a proper solution. I need Gunnar to give his opinion on this.)
- This pull request will be accompanied with a nice test case in another pull request.
- Code has only been tested with Python 2.7.

Once this has been merged, I will work on the SPARQL 1.1 Protocol implementation, i.e. query and update.

This pull request is a part of #1.

The specification can be found here: http://www.w3.org/TR/sparql11-http-rdf-update/