Use SPARQLWrapper for updates in SPARQLStore

New versions of SPARQLWrapper now support SPARQL updates too. Therefore
the methods inherited from SPARQLWrapper should be used for updates
instead of building the request manually.

Drawback:
The user can no longer set whether to send updates as
application/sparql-update or application/x-www-form-urlencoded.
This feature should be introduced in SPARQLWrapper and the old
behavoiur in SPARQLStore restored.
Cleanups wrt. to usage of SPARQLWrapper

Mainly, we don't need to handle our own connection anymore. The
keep alive option of SPARQLWrapper is activated. Some other small
changes to improve consistency.
Cleaned up some comments in SPARQLStore

Removed empty/unnecessary comments and improved existing one.
Merge branch 'master' into sparqlstore-sparqlwrapper

Support for named graphs in the SPARQLUpdateStore is required for
further development.

Conflicts:
	rdflib/plugins/stores/sparqlstore.py
Restored functionality of option `postAsEncoded`

Thanks to the newly introduced method `setUpdateMethod` of the
SPARQLWrapper, the user can again choose whether to send SPARQL Updates
as `application/sparql-update` or as `application/x-www-form-urlencoded`.
Method SPARQLWrapper.setUpdateMethod renamed to setRequestMethod

This should work with SPARQLWrapper 1.6.1.
Merge pull request #397 from RDFLib/sparqlstore-sparqlwrapper

SPARQLStore: Use SPARQLWrapper for updates

SPARQLStore was using its own implementation to send SPARQL Update requests.
Now, it uses SPARQLWrapper's update mechanism in the same way it already uses its query mechanism.