title: How to merge two dataframe in pandas to replace nan
body: <p>I want to do this in pandas:</p>
  <p>I have 2 dataframes, A and B, I want to replace only NaN of A with B values.</p>
  <pre><code>A                                                
  2014-04-17 12:59:00  146.06250  146.0625  145.93750  145.93750
  2014-04-17 13:00:00  145.90625  145.9375  145.87500  145.90625
  2014-04-17 13:01:00  145.90625       NaN  145.90625        NaN
  2014-04-17 13:02:00        NaN       NaN  145.93750  145.96875
  
  B
  2014-04-17 12:59:00   146 2/32   146 2/32  145 30/32  145 30/32
  2014-04-17 13:00:00  145 29/32  145 30/32  145 28/32  145 29/32
  2014-04-17 13:01:00  145 29/32        146  145 29/32        147
  2014-04-17 13:02:00        146        146  145 30/32  145 31/32
  
  Result:
  2014-04-17 12:59:00  146.06250  146.0625  145.93750  145.93750
  2014-04-17 13:00:00  145.90625  145.9375  145.87500  145.90625
  2014-04-17 13:01:00  145.90625       146  145.90625        147
  2014-04-17 13:02:00        146       146  145.93750  145.96875
  </code></pre>