v 1.6.1 breaks delete API

[BezBartek]

Version 1.6.1 adds to method _delete new keyword, resKey with different default (None), than _req method has ("results"). That of course would break API, and forces to provide "results" as resKey parameter, to make _delete work. But delVerticesById that wraps _delete do not inherit that interface (it doesn't accept resKey param). So in fact, you no longer can use delVerticesById (Key error! cuz there is no deleted_vertices in response dict)

• It is wrong default on _delete method, it should be exactly same as on _req.
• It is unclear, why _delete method has that parameter anyway. I would advise dropping it.