fast cross correlation function/method
this is bordering on not being the domain of pandas, but i think a library that is advertised at being good at time series should include this functionality

the numpy version is dog slow

something like 

``` python
s.xcorr(s2, lags=100, scaling='normalized') # first 100 lags, scaled to be -1 <= 0 <= 1
df.xcorr() # all non repeating pairs of series_i, series_j xcorrs
df.xcorr(s2) # xcorr of all series in a frame with s2
```
