enh request: ``describe`` for Factor, string fields, etc
I wonder if `Factor` and things that are Factor-coercible should have some of the methods that `collections.counter` items have (in particular, `most_common`). 

 The R `summary` output on such a thing is:

```
id        
```

id1:            45577  
id2:            16582  
id3:             12446  
(Other):      340115  
