Merge SPARQL functionality

[LukasBluebaum]

In qa.commons, there are 2 different ways to fire queries to an endpoint, SPARQL and SPARQLExecutor. This becomes tedious to keep clean, especially when both share the same (hardcopied) code.
Consider merging the functionality in favor of SPARQL.

[RicardoUsbeck]

Makes sense.
@Aklakan as an expert, do you have any recommendation which to keep?

[LukasBluebaum]

There was already a TODO in the code that SPARQL should be kept. I guess thats because it also supports caching and the other one doesn't.