Language-specific translator groups

Hi,

I have tried to add a new option that allows you to create language-specific groups of translators.

If you add this to your settings:

```python
ROSETTA_LANGUAGE_GROUPS = True
```

Adding users to a `translators` group will give them access to Rosetta, but not to any actual languages – the list will be empty. For each language, that you have, you need to create a similarly named group `translators-XX` for XX being the language identifier (e.g. `fr`, `fr_FR` or `fr_FR.utf8`). Then you can add each translator to the group(s), that they need access to.

Staff users will have access to all languages regardless of groups.

I have not created a test for this (there didn't seem to be a test for the basic translators group to start from), but can probably do so if it pleases the crowd.