remove the global stream object.

Since everything is driven by the test case, it's better to have a reference to
the streamer there, to be passed to the different parts that need to send
information (the requests, session, webtest or the websocket object).
Rename StreamCollector in Collector.
Refactoring

- The concept of streams had been replaced by collectors and outputs.
- No more global streams, it allows to have multiple instances of the same
  runner class with different outputs, for instance.
- The APIs of the collector changed a bit: no more "push" method, but we have
  add_hits, addErrors, addFailure, etc. methods for everything.