read_csv treats both inf and -inf as very large negative number
from [mailing list](https://groups.google.com/forum/#!topic/pydata/Wy3eBEnoDrM), same discussion led to #2026

``` python

In [42]: data = """\
   ....: ,A
   ....: a,inf
   ....: b,-inf
   ....: """

In [43]: df = pd.read_csv(StringIO(data), index_col=0)

In [44]: df
Out[44]: 
                     A
a -9223372036854775808
b -9223372036854775808

In [45]: df.dtypes
Out[45]: A    int64
```
