title: Combining multiple data frames with different number of columns
body: <p>I'm preparing the stats for currency values, I have currency values for every day, every year is in an individual excel-file. I've managed to read <code>xls</code> files, but I want to merge the data frames for every year. They look like:</p>
  
  <pre><code>Date      EUR   SEK   NOR   PLN   ROM   SKR
  1-1-1998  2,21  1,23  1,13  2,99  9,12  6,17
  2-1-1998  1,13  12,2  2,11  9,22  1,11  2,33
  </code></pre>
  
  <p>But some of them has more colums (additional currency), like:</p>
  
  <pre><code>Date      EUR   SEK   NOR   PLN   ROM   SKR   JKC
  1-1-1998  2,21  1,23  1,13  2,99  9,12  6,17  1,11
  2-1-1998  1,13  12,2  2,11  9,22  1,11  2,33  2,13
  </code></pre>
  
  <p>I just want to merge them this way:</p>
  
  <pre><code>Date      EUR   SEK   NOR   PLN   ROM   SKR
  1-1-1998  2,21  1,23  1,13  2,99  9,12  6,17
  2-1-1998  1,13  12,2  2,11  9,22  1,11  2,33
  Date      EUR   SEK   NOR   PLN   ROM   SKR   JKC
  1-1-1998  2,21  1,23  1,13  2,99  9,12  6,17  1,11
  2-1-1998  1,13  12,2  2,11  9,22  1,11  2,33  2,13
  </code></pre>
  
  <p>etc. </p>
  
  <p>I tried to use <code>merge()</code>, but it treated identical values as one, <code>rbind</code> doesn't work because there are different numbers of columns. How should I combine those data frames?</p>