read_csv and missing data issue
``` python
In [69]: data = """\
   ....: A,B,C
   ....: foo,,
   ....: ,bar,
   ....: ,,baz
   ....: """

In [70]: pd.read_csv(StringIO(data))
---------------------------------------------------------------------------
CParserError                              Traceback (most recent call last)
...
CParserError: Error tokenizing data. C error: EOF inside string starting at line 2
```

.

``` python
In [79]: data = """\
   ....: A,B
   ....: foo,
   ....: ,bar
   ....: """

In [80]: pd.read_csv(StringIO(data))
Out[80]:
         A   B
0      foo NaN
1  bar("") NaN
```

This is fine

``` python
In [67]: data = """\
   ....: A,B,C
   ....: ,,foo
   ....: ,bar,
   ....: baz,,
   ....: """

In [68]: pd.read_csv(StringIO(data))
Out[68]:
     A    B    C
0  NaN  NaN  foo
1  NaN  bar  NaN
2  baz  NaN  NaN
```
