read_csv: chunksize clashes with nrows
If nrows are determined, the use of chunksize option does not create a TextFileReader object.

``` python
reader = pd.read_table(fname, sep=',',chunksize=4, na_values=['null'],
                       nrows=20)
type(reader)
```

``` bash
pandas.core.frame.DataFrame
```

My suggestion:
- Either put into docs that they are not to be used concurrently
- or it's a feature request that they can.

I would find it useful to get chunks of x size, but only for the first n row of a huge file.
