Fix Issue #21. Ensure context is always created by trying to add it in _yapp_callback. There may be conditions where yapp_callback is called with a thread which we have not created a context entry.
Fix Issue #21. Ensure context is always created by trying to add it in _yapp_callback. There may be conditions where yapp_callback is called with a thread which we have not created a context entry.

sumerc/yappi,sumerc/yappi