pearson corr can throw IndexError
I just got this from a ipython parallel run (so unfortunately no reproduceable case):

```
IndexError                               Traceback (most recent call last)<string>in <module>()
C:\data\phd\montecarlo-paper\functions.py in simulate(inputs)
C:\portabel\Python27\lib\site-packages\pandas\core\frame.pyc in corr(self, method)
   3355                valid=mask[i]&mask[j]
   3356                if not valid.all():
-> 3357                     c=corrf(ac[valid],bc[valid])
   3358                else:
   3359                    c=corrf(ac,bc)
C:\portabel\Python27\lib\site-packages\pandas\core\nanops.pyc in _pearson(a, b)
    328
    329    def _pearson(a,b):
--> 330         return np.corrcoef(a,b)[0,1]
    331    def _kendall(a,b):
    332        return kendalltau(a,b)[0]
IndexError: invalid index
IndexError(invalid index)
```

(i had to do some S&R to get the stacktrace into this form, so hopefully I didn't reformat/remove anything)

I'm not sure under which circumstance np.corrcoef does not return an array, but if so pandas should return "nan" in that case.
