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- fix issue 20: support argument-free post mortem

Changes between 0.7.1 and 0.7.2

- don't crash with newer versions of ``py.test``

Changes between 0.7 and 0.7.1

- The ``pp`` (pretty print) command now uses the actual width of the terminal,
  instead of hardcoding 80 columns

- py.test and unittest internal frames are hidden by default (type ``help
  hidden_frames`` for more info)

- don't crash if ``py`` is installed but ``py.test`` is not