BUG: Unable to parse monthly-string in datetime.pxd
Travis build looks fail with following error.

```
  File "/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/pandas/tseries/tests/test_period.py", line 1976, in test_range_slice_outofbounds
...
 File "datetime.pxd", line 137, in datetime._string_to_dts (pandas/tslib.c:52202)
ValueError: Unable to parse 2013/06
```

The failure is caused by test in `DatetimeIndex`, strangely `PeriodIndex` has no problem.

I don't know where `datetime.pxd` is derived from, but one assumption is the `_string_to_dts` uses todays date (31) in parsing process.

```
import pandas as pd

didx = pd.DatetimeIndex(start='2013/10/01', freq='D', periods=10)
pidx = pd.PeriodIndex(start='2013/10/01', freq='D', periods=10)

for idx in [didx, pidx]:
    df = pd.DataFrame(dict(units=[100 + i for i in range(10)]), index=idx)
    for key in ['2013/{0}'.format(i) for i in range(1, 13)]:
        try:
            df[key:'2013/03']
        except ValueError, e:
            print(type(idx), key, e)

# will prints
(<class 'pandas.tseries.index.DatetimeIndex'>, '2013/2', ValueError('Unable to parse 2013/2',))
(<class 'pandas.tseries.index.DatetimeIndex'>, '2013/4', ValueError('Unable to parse 2013/4',))
(<class 'pandas.tseries.index.DatetimeIndex'>, '2013/6', ValueError('Unable to parse 2013/6',))
(<class 'pandas.tseries.index.DatetimeIndex'>, '2013/9', ValueError('Unable to parse 2013/9',))
(<class 'pandas.tseries.index.DatetimeIndex'>, '2013/11', ValueError('Unable to parse 2013/11',))
```
