Add fast pairwise rolling correlation function
@idrisr

``` python
def rolling_corr_pairwise(df, window):
   all_results = {}

   for col, left in df.iteritems():
       all_results[col] = col_results = {}
       for col, right in df.iteritems():
           col_results[col] = rolling_corr(left, right, window)

   return Panel(all_results)
```
