title: Merge two CSV's with unique columns in python
body: <p>I have two CSV files representing data from two different years.  I know how to do the basic merging using csvwriter and dictkeys,  but the problem lies here:  while the CSVs have mostly shared column headers, each may have unique columns.  If a species was caught in one year but not the other, that column would only be present in that year.  How can I merge the new data to the old data, creating new columns and padding the old data with zero in those columns? </p>
  
  <p>File 1:     <code>"Date","Time","Species A","Species B", "Species X"</code></p>
  
  <p>File 2:     <code>"Date","Time", "Species A", "Species B", "Species C"</code></p>
  
  <p>I need the end result to be one csv with this header:
         "<code>Date","Time","Species A","Species B", "Species C", "Species X"</code></p>