Bugfix for "DROP GRAPH"

no additional execution of original query on SPARQL endpoint since the graph was alredy dropped by R43ples in purgeGraph()

additional comments included

@stephenhensel: does this fix #18 for you?
Merge branch 'master' of https://github.com/plt-tud/r43ples
added createTestDataset HTTP metod which creates a test data set

fixes bugfix in testing method
fix coverity issue