Introducing GraphDatabaseService.execute(...)

Enables executing cypher queries without having to keep track of an ExecutionEngine instance, instead the database keeps the ExecutionEngine internally.
@thobe  That doesn't work either. The statement (e.g. `EXPLAIN ...`) was executed, and did as requested - returned an explanation of how the specified query would be evaluated.

This might be a good one for @akollegger to ponder.