Adapting to RDFLib API changes wrt. ConjunctiveGraph

The behaviour of ConjunctiveGraph wrt. the default graph and empty
graphs can now be configured, therefore the endpoint uses now
works ConjunctiveGraphs.

Also fixed some mistakes.
Fix starting via commandline

The endpoint can now again be started via the command line using

    python -m rdfextras_web.endpoint <RDF-file>

If the RDF-file is omitted, the endpoint starts with an empty Dataset.
Importing the graphstore protocol testcase

This is a copy of
http://www.w3.org/2009/sparql/docs/tests/data-sparql11/http-rdf-update/

The idea is to parse this HTML page and run the tests it contains.
Fixed w3c's testcase (hrmpf!)

Fixed missing Accept headers

Uncommented a wrong test:
This request should fail according to the testcase. To me and to my
implementation, this request looks perfectly valid.

Fixed relative URLs as path argument

Fixed response codes:
Changed several response codes. Mostly from 200 to 204 because there
is no response body.

Fixed bad turtle syntax

Added comments indicating the changes (look for 'rdflib').
Testcase parser for graphstore tests

This simply parses the HTML file describing the tests and then runs
them accordingly.