forward rolling functions
https://groups.google.com/forum/#!msg/pydata/LLPnzMfDlSg/OpK-mHJzJwYJ
http://stackoverflow.com/questions/22820292/how-to-use-pandas-rolling-functions-on-a-forward-looking-basis

Applying to reverse Series and reversing could work on all (?) rolling functions, I think sometimes can just do on values array, a kwarg would be nice.

```
pd.rolling_(s[::-1])[::-1]
pd.Series(pd.rolling_(s.values[::-1])[::-1), s.index, name=s.name)  # faster?
```

(OP says that this doesn't actually work in some cases: http://stackoverflow.com/questions/22820292/how-to-use-pandas-rolling-functions-on-a-forward-looking-basis/22820689?noredirect=1#comment34813442_22820689... ?)
