TwitterStream bugfixes

* do not run disabled `TwitterStreamAgents`
* calling `sleep` within the EM loop blocks the EM thread and thereby
  blocks the reload callback and proper shutdown of the threaded worker
* stopping the EM loop from outside of the `EM.run` blocks does not seem
  to work reliably, using a timer and checking if shutdown was requested
  works as expected
* removed unnecessary sleep at the end of the EM loop which would delay
  the shutdown of the thread
Merge pull request #442 from dsander/twitterstream-fixes

TwitterStream bugfixes