to_string() returning None
DataFrame.to_string() is printing directly to console and not returning a string :

``` Python
>>> import pandas
>>> df = pandas.DataFrame(range(5))
>>> df
   0
0  0
1  1
2  2
3  3
4  4
>>> view_repr = repr(df)
>>> view_to_string = df.to_string()
   0
0  0
1  1
2  2
3  3
4  4
>>> view_repr == view_to_string
False
```

For large DataFrame, we have to use to_string() to get a tabular view.
But the return value is not set (None) and can't be retrieved :

``` Python
>>> view_repr
'   0\n0  0\n1  1\n2  2\n3  3\n4  4\n'
>>> view_to_string
>>>
```

It would be usefull to get a string (or StringIO) returned by DataFrame.to_string()
