Feature/groupby repr ellipses 1135
- [ ] closes #1135 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Currently one test is failing:

```
    def test_groups(self, df):
        grouped = df.groupby(['A'])
        groups = grouped.groups
        assert groups is grouped.groups  # caching works
```

I'm not sure what exactly this test is checking for. Is this a behavior that needs to be kept?