Add --pdb option; invokes pdb on exception

Useful for debugging. Looks like this:

    $ honcho export . supervisord  --pdb
    [Errno 13] Permission denied: '/var/log/export_test'
    Traceback (most recent call last):
      File "/Users/marca/dev/git-repos/honcho/honcho/command.py", line 253, in main
        COMMANDS[args.command](args)
      File "/Users/marca/dev/git-repos/honcho/honcho/command.py", line 106, in command_export
        export.export()
      File "/Users/marca/dev/git-repos/honcho/honcho/export/base.py", line 86, in export
        self._mkdir(self.options.log)
      File "/Users/marca/dev/git-repos/honcho/honcho/export/base.py", line 47, in _mkdir
        .format(directory))
    CommandError: Can not create /var/log/export_test
    > /Users/marca/dev/git-repos/honcho/honcho/export/base.py(47)_mkdir()
    -> .format(directory))
    (Pdb) directory
    '/var/log/export_test'
    (Pdb) bt
      /Users/marca/dev/git-repos/honcho/honcho/command.py(253)main()
    -> COMMANDS[args.command](args)
      /Users/marca/dev/git-repos/honcho/honcho/command.py(106)command_export()
    -> export.export()
      /Users/marca/dev/git-repos/honcho/honcho/export/base.py(86)export()
    -> self._mkdir(self.options.log)
    > /Users/marca/dev/git-repos/honcho/honcho/export/base.py(47)_mkdir()
    -> .format(directory))
    (Pdb)