Poisson (exponential) window for pandas.Series.rolling
#### Problem Description
In [`pandas.Series.rolling`](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.rolling.html), the [SciPy window type `exponential`](https://docs.scipy.org/doc/scipy/reference/generated/scipy.signal.windows.exponential.html#scipy.signal.windows.exponential) has not been implemented. This `win_type` would be extremely useful for analysing [biological processes](https://en.wikipedia.org/wiki/Time_constant#Time_constants_in_neuroscience). It should be pretty easy to add.

Please, note that despite the similar name, the proposed method is mathematically different from the implemented [`pandas.Series.ewm`](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.ewm.html) method.

#### Enhancement Request
Taking for example `df.rolling(51, win_type='exponential', tau=10)`, the implemented SciPy window would be the following asymmetrical window:
```
from scipy import signal
window = 51
tau = 10
w = signal.exponential(window, tau=tau, sym=False, center=0)
array([1.        , 0.90483742, 0.81873075, 0.74081822, 0.67032005,
       0.60653066, 0.54881164, 0.4965853 , 0.44932896, 0.40656966,
       0.36787944, 0.33287108, 0.30119421, 0.27253179, 0.24659696,
       0.22313016, 0.20189652, 0.18268352, 0.16529889, 0.14956862,
       0.13533528, 0.12245643, 0.11080316, 0.10025884, 0.09071795,
       0.082085  , 0.07427358, 0.06720551, 0.06081006, 0.05502322,
       0.04978707, 0.0450492 , 0.0407622 , 0.03688317, 0.03337327,
       0.03019738, 0.02732372, 0.02472353, 0.02237077, 0.02024191,
       0.01831564, 0.01657268, 0.01499558, 0.01356856, 0.01227734,
       0.011109  , 0.01005184, 0.00909528, 0.00822975, 0.00744658,
       0.00673795])
```


#### Expected Output
```
>>> df = pd.DataFrame({'B': [1, 0, 0, 0, 0]})
>>> df.rolling(51, win_type='exponential', tau=10)
     B
0  1.0
1  0.90483742
2  0.81873075
3  0.74081822
4  0.67032005
```
