Dutch translation

This pull request adds the dutch translation (nl). The polish locale commit includes both django.po and django.mo. When I commited the dutch translation I noticed git was told to ignore *.mo files, but afaik most projects include this binary file, so I removed that gitignore line as well.

I haven't added the following line to `MANIFEST.in` because it needs some testing but almost sure something along these lines needs to be added as well:

```
recursive-include session_activity/locale *
```