read_csv with iterator=True does not seem to work as expected without chunksize
Hi there,

I tested:

``` python
data = """A,B,C
foo,1,2,3
bar,4,5,6
baz,7,8,9
"""
reader = pd.read_csv(StringIO(data), iterator=True)
```

I thought that I could do:

```
for row in reader:
    print(row)
```

since `reader` is iterable. Unfortunately, it calls the generator `TextFileReader.__iter__`:

``` python
def __iter__(self):
    try:
        while True:
            yield self.read(self.chunksize)
    except StopIteration:
        pass
```

where `self.chunksize` is `None`. Maybe set `self.chunsize` to 1 when it's not
defined and there is `iterator=True`. I'll propose a patch as soon as possible
--- today or tomorrow.

Best regards,
Damien G.
