BUG: Series.median() treats NaT as INT_MIN
Fixing this will be easier once numpy/numpy#5222 is fixed.

``` python
In [1]: import pandas as pd

In [2]: pd.Series([0, pd.tslib.iNaT], dtype='m8[ns]')
Out[2]: 
0   0 days
1      NaT
dtype: timedelta64[ns]

In [3]: _.median()
Out[3]: 
0   -53375 days, 23:53:38.427388
dtype: timedelta64[ns]
```
