StreamCapturer should die if main process crashes.
Merge pull request #4498 from takluyver/daemon-streamcapturer

Daemon StreamCapturer

The StreamCapturer should die if the main thread crashes. On Shiningpanda, a failure in another nose plugin has been causing the tests to hang, because the main thread exits, but the StreamCapturer thread is still alive.

Under normal conditions, the thread will still be shut down cleanly - it will only die a messy death if the main thread does.