Code review comment for lp:~danilo/launchpad/bug-459132

Thank you for getting this done. The branch looks good to me, apart from the few things which we discussed on IRC:

- Use the Storm In operator in potemplate.py
- Fix the docstring for CreditsFixer
- Rewrite the test for getPOFilesWithTranslationCredits to use sample data that does not have dummy translation credits messages set as makePOTMsgSet will only create POTMsgSets wich already have them.

Thank you. ;)