df.groupby('key').groups printed all: problem with large arrays
When you are working with a large array, it is not printed out in its entirety in the console, but when you have grouped them with `groupby('key')`, the groups are all printed out. Would it be possible to also restrict the output of `groupby('key').groups` to eg the first and last groups?

I was working with a rather large dataframe (around 80000 rows), and first it took a long time to print it all, and second the console got stuck for a while (but that could also be an issue with spyder).
I know it is not very useful to print out the groups, but I was curious to see what it would look like, but it was not a very good idea with such a large array.


```python
>>> import numpy as np
>>> import pandas as pd
>>> df = pd.DataFrame(np.random.randn(100000, 4), columns=list('abcd'))
>>> df['g'] = np.random.randint(0, 100, 100000)
>>> df.groupby('g').groups  # this repr should be truncated
```