title: Combining two time series in pandas
body: <p>Apologies if this is obviously documented somewhere, but I'm having trouble discovering it.  I have two TimeSeries with some overlapping dates/indices and I'd like to merge them.  I assume I'll have to specify which of the two series to take the values from for the overlapping dates.  For illustration I have:</p>
  
  <pre><code>s1:
  2008-09-15    100
  2008-10-15    101
  
  s2:
  2008-10-15    101.01
  2008-11-15    102.02
  </code></pre>
  
  <p>and I want:</p>
  
  <pre><code>s3:
  2008-09-15    100
  2008-10-15    101
  2008-11-15    102.02
  </code></pre>
  
  <p>or</p>
  
  <pre><code>s3:
  2008-09-15    100
  2008-10-15    101.01
  2008-11-15    102.02
  </code></pre>