Standard deviation error?
Hey Wes, I think there's a bug in your implementation of standard deviation in 0.8.0.dev-7085b94. In an arbitrary dataset, if I use df.groupby(df.col).std(), I get NaNs in some records. If I use groupby([df.col]).apply(lambda x: np.std(x)), the records are correct (0.000000). 

Haven't had a chance to dig into it deeper, but prima facie this seems like a bug. Let me know what other info you need. 

Cheers,
Peter
