Series.skew() for constant series returns inconsistent values
I'm not sure if this is a bug, but I'v noticed a strange behavior of skew() and kurt() on constant series:

``` python
#skew:
pd.Series(3075.3, np.arange(300)).skew() #returns -105683727.49921818
pd.Series(3075.2, np.arange(300)).skew() #returns nan
pd.Series(3075.5, np.arange(300)).skew() #returns 0

#kurt:
pd.Series(3075.3, np.arange(300)).kurt() #returns 9305221026420888.0
pd.Series(3075.2, np.arange(300)).kurt() #returns -7459707710545691.0
pd.Series(3075.5, np.arange(300)).kurt() #returns 0
```

(using pandas 0.17.1 (from anacondas) on a 64bit intel windows 7 pc)
