ENH: Add "select all" option for duplicates
How would people feel about me adding a "select_all" option to duplicates / duplicated that returns all duplicated rows, not just all but first or all but last?

I often check to see if I have duplicates by some primary key, and if I do, I then want to look at ALL rows with the same primary key. Right now, duplicates / duplicated won't do so and I need to use the following hack:

```
df[df.duplicated('key',take_last=True) | df.duplicated('key')]
```

It seems I"m not the only person with this issue: 
http://stackoverflow.com/questions/26244309/how-to-analyze-all-duplicate-entries-in-this-pandas-dataframe
http://stackoverflow.com/questions/14657241/how-do-i-get-a-list-of-all-the-duplicate-items-using-pandas-in-python
http://stackoverflow.com/questions/23667369/drop-all-duplicate-rows-in-python-pandas

Happy to write PR if people support. 
