Extension "Weeks" not working due to a syntax error

If i execute the `Weeks` extension, i get the error message:

```
Parsing log for CVSAnalY (git)
Warning: Detected empty branch 'master', it'll be ignored
Warning: Detected empty branch 'markdown-readme', it'll be ignored
Warning: Detected empty branch 'ignore-config', it'll be ignored
Warning: Detected empty branch 'patch-1', it'll be ignored
Warning: Detected empty branch 'libresoft-utils', it'll be ignored
Database looks empty, removing cache file /Users/andygrunwald/.cvsanaly2/cache/https:__github.com_andygrunwald_CVSAnalY.git
Executing extensions
Executing extension Weeks
/Users/andygrunwald/Development/CVSAnalY/pycvsanaly2/extensions/Weeks.py:102: Warning: Unknown table 'cvsanaly.weeks'
  cursor.execute ("DROP TABLE IF EXISTS weeks")
```

This error message make no sense, because of the `IF EXISTS` part in query.
But this message occurs because the Weeks extension got a syntax error in the Weeks.py

This pull requests will fix this error and make the Weeks extension working again.