read_csv with usecols and implicit index columns
Pandas 0.12
When specifying usecols with read_csv it seems impossible to use an implicit (no header) index column

Example: 

``` python
>>> data = 'a,b,c\n4,apple,bat,5.7\n8,orange,cow,10'
>>> pd.read_csv(StringIO(data))
        a    b     c
4   apple  bat   5.7
8  orange  cow  10.0
>>> pd.read_csv(StringIO(data), usecols=['a', 'b'])
        a    b     
0   apple  bat
1  orange  cow
>>>
```

Where you would prefer/expect this:

``` python
>>> pd.read_csv(StringIO(data), usecols=['a', 'b'])
        a    b     
4   apple  bat
8  orange  cow
```

Specifying index_col = 0 would give:

``` python
>>> pd.read_csv(StringIO(data), usecols=['a', 'b'], index_col = 0)
          b     
apple   bat
orange  cow
```

So this would only work if we explicity pass the column to be used as an index to usecols. However, we cannot explicitly pass an implicit index column to usecols as the index column is not named (it has no header).
