Fixed optional parameter exclude.

Exception raised when used without exclude:
```
bin/i18ndude rebuild-pot --pot /Users/lukas/projects/opengever.core.dev/opengever/trash/locales/opengever.trash.pot --create opengever.trash ./opengever/trash
Traceback (most recent call last):
  File "bin/i18ndude", line 31, in <module>
    i18ndude.script.main()
  File "/Users/lukas/Plone/eggs/i18ndude-3.3.0-py2.7.egg/i18ndude/script.py", line 612, in main
    errors = arguments.func(arguments)
  File "/Users/lukas/Plone/eggs/i18ndude-3.3.0-py2.7.egg/i18ndude/script.py", line 210, in rebuild_pot
    exclude = tuple(arguments.exclude.split())
```