title: Count number of non-NaN entries in every column of Dataframe
body: <p>I have a really big DataFrame and I was wondering if there was short (one or two liner) way to get the a count of non-NaN entries in a DataFrame. I don't want to do this one column at a time as I have close to 1000 columns. </p>
  
  <pre><code>df1 = pd.DataFrame([(1,2,None),(None,4,None),(5,None,7),(5,None,None)], 
                      columns=['a','b','d'], index = ['A', 'B','C','D'])
  
      a   b   d
  A   1   2 NaN
  B NaN   4 NaN
  C   5 NaN   7
  D   5 NaN NaN
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>a: 3
  b: 2
  d: 1
  </code></pre>