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use traceback.formation_exception_only to be more error-resistant when displaying fancy tracebacks with unicode strings

reimplement source colorization :
  - use --color if you want your traceback to be colorized
  - ipython is used (as in debugger.py) rather than pygments directly
  - only python source lines are passed to the colorizer (not all the
    formatted traeceback)
  - the filenames are also colorized

This patch also provides a new feature: when --verbose is used,
local variables are displayed.

unittest checks if a __unittest variable is found in the frame's
global variables to decide whether or not this frame is relevant
in the final traceback.