OR-513 Refactor the notification module

Replace directives by a factory.
OR-513 Refactor websocket factory
OR-513 Refactor activity streams notification
OR-513 Refactor conference notification
OR-513 Add the directive for the api notification
OR-513 Add tests

Tests added for activity stream, api and conference notification.
Merge pull request #355 in OR/rse from ~RPIGNOLET/rse:OR-513 to master

* commit '9e74ed8942dd44200978b0a6c9268ec37341b151':
  OR-513 Add tests
  OR-513 Add the directive for the api notification
  OR-513 Refactor conference notification
  OR-513 Refactor activity streams notification
  OR-513 Refactor websocket factory
  OR-513 Refactor the notification module