title: pandas ewm and rolling methods to not fill in NAs
body: <p>Is there a parameter for ewm and rolling methods to not fill in the NAs?</p>
  
  <pre><code>&gt;&gt;&gt; A = pd.Series([1,2,3,4,np.nan,5,6])
  &gt;&gt;&gt; A
  0    1.0
  1    2.0
  2    3.0
  3    4.0
  4    NaN
  5    5.0
  6    6.0
  dtype: float64
  &gt;&gt;&gt; eA = A.ewm(alpha = 0.5, ignore_na=True).mean()
  &gt;&gt;&gt; eA
  0    1.000000
  1    1.666667
  2    2.428571
  3    3.266667
  4    3.266667 # I want this to be NA, don't fill in
  5    4.161290
  6    5.095238
  dtype: float64
  </code></pre>
  
  <p>Of course this is easily solved by </p>
  
  <pre><code>eA[A.isnull()] = np.nan
  </code></pre>
  
  <p>But this takes some unnecessary running time, and the need to think of a variable name for every rolling function is bothersome when you have more than a few.</p>