API: meaning of min_periods for ewm*() functions?
The interpretation of `min_periods` in the `ewm*()` functions seems rather odd to me. For example (in 0.14.1):

```
In [19]: x
Out[19]:
0     0
1   NaN
2   NaN
3   NaN
4     4
5   NaN
6     6
dtype: float64

In [20]: ewma(x, com=3., min_periods=2)
Out[20]:
0         NaN
1         NaN
2    0.000000
3    0.000000
4    2.285714
5    2.285714
6    3.891892
dtype: float64
```

The way it works, is it finds the first non-`NaN` value (`0` in the example above) and then makes sure that the `min_periods` entries (`min_periods-1` in 0.15.0, per https://github.com/pydata/pandas/pull/7898) in the result starting at that entry are `NaN`. Does it make any sense that the result has entry `0` set to `NaN`, but entries `2` and `3` (and `1` in 0.15.0) set to `0.0`?

I would have thought that the values to be explicitly `NaN`ed would be those determined by `x.notnull().cumsum() < min_periods`. This would be consistent with the meaning of `min_periods` in the `rolling_*()` and `expanding_*()` functions.

CC'ing @snth and @jaimefrio, in case they have opinions.
