API: rolling_* functions should have default window arg of None
I could be missing an easier way, but a lot of times I want to apply a rolling function to something like a groupby. This isn't the easiest thing in the world since the rolling functions take 2 arguments and `groupby.apply` is expecting a function with one argument. Maybe there's some way with passing in `args` to `groupby.apply`, but I can never remember what exactly get passed where, and what needs to be in tuples.

The most intuitive way (for me) is to make the function being applied, `f`, a function of one argument (the grouped values). This is difficult/impossible with the rolling funcs since there are 2 positional arguments and we want to `partial` the second. You have go through hoops with flipping function arguments, `partial`ing, and flipping back. meh...

I've got a patch that would allow

``` python
In [1]: df = pd.DataFrame(np.random.randn(100, 4))

In [2]: df['g'] = np.random.randint(0, 3, size=(100))

In [3]: from functools import partial

In [4]: f = partial(pd.rolling_mean, window=4)

In [5]: df.groupby('g').apply(f)
```

This is fully backwards compatible since it just raises if window is None. Any reason not to?
