PEP 454

* rename disable/enable/is_enabled() to stop/start/is_tracing()
* add disable/enable/is_enabled() which are temporary, and disable() doesn't
  clear traces
* a traceback now always contains a least 1 frame, use (('<unknown>', 0),) if
  the traceback cannot read or if the traceback limit is 0
* write more documentation on filter functions