ENH: add "adjust" parameter to ewmvar(), ewmstd(), ewmvol(), ewmcov(), and ewmcorr()
Currently `ewma()` has an `adjust` parameter (affecting how the weights are calculated) whereas all the other `ewm*()` functions do not. When they call `ewma()`, `adjust` is implicitly set to its default value, `True`. This is probably fine in most cases, but if `ewma()` is going to expose the `adjust` parameter, the other `ewm*()` should as well.

For clarity, here's the description of the `adjust` parameter in `_ewm_notes`:

```
When adjust is True (default), weighted averages are calculated using weights
    (1-alpha)**(n-1), (1-alpha)**(n-2), ..., 1-alpha, 1.

When adjust is False, weighted averages are calculated recursively as:
    weighted_average[0] = arg[0];
    weighted_average[i] = (1-alpha)*weighted_average[i-1] + alpha*arg[i].
```

On a related note, I'm not sure what the policy/practice is for reordering parameters, but if it's not too strict I'd like to reorder the `ewm*()` parameters just a little so that (i) `freq` and `how` are next to each other, and (ii) `adjust` and `ignore_na` are next to each other.
