read_csv dateparser empty date's are assigned today's date
Hi, 

``` python
import StringIO
s = StringIO.StringIO("Date, test\n2012-01-01, 1\n,2")
pd.read_csv(s, parse_dates=["Date"])


    Date     test
0    2012-01-01 00:00:00     1
1    2012-11-15 00:00:00     2
```

second row should be None.

is this the default behaviour? 
this is a quick fix for me

``` python
from dateutil.parser import parse as dateparser
def parse_date(d):
    if d.strip():
        return dateparser(d)

pd.read_csv(s, parse_dates=["Date"], date_parser=parse_date)


using pandas 0.9.1
```
