Support url variants and https

* the api can use a path like '/api/v2/executions' or a full url like
  'https://ci2.zhdk.ch/api/v2/executions' for refs; this now supported

* support https or http (still default) by specifying the schema with
  the host parameter


Most is said in the commit message above. I hat to refactor all the get requests too because you would run into trouble if you put user/password into the url if the api would also use full urls. Ah, the test is broken again. Sorry for that.