remove the subscription record upon successful unsubscribe
unit test for PubSub subcription records removal
Merge branch 'hotfix/3241'

Close #3241
Merge branch 'master' of git://github.com/quocvu/zf2 into hotfix/3295
[#3295] Fixed tests and logic flow

- Fixed the unit tests for the unsubscribe functionality. The table
  gateway was expecting the "delete" method now (not "update"), with
  only the "id" member in the array argument; additionally, we needed to
  test the return value, so I added a "will" clause.
- Altered the workflow of Callback to use a switch() statement for the
  "hub_mode"; this will make adding additional cases in the future
  easier, and allows us to raise an exception on an invalid mode.
Merge branch 'hotfix/3295' into develop

Forward port #3295