title: Complete column with group_by and complete
body: <p>I've got a little problem using dplyr <code>group_by</code> function.
  After doing this :</p>
  <pre><code>datasetALL %&gt;% group_by(YEAR,Region) %&gt;% summarise(count_number = n()) 
  </code></pre>
  <p>here is the result :</p>
  <pre><code>YEAR Region count_number
  &lt;int&gt;  &lt;int&gt;        &lt;int&gt;
  1   1946      1            2
  2   1946      2            3
  3   1946      3            1
  4   1946      5            1
  5   1947      3            1
  6   1947      4            1
  </code></pre>
  <p>I would like something like :</p>
  <pre><code>YEAR Region count_number
  &lt;int&gt;  &lt;int&gt;        &lt;int&gt;
  1   1946      1            2
  2   1946      2            3
  3   1946      3            1
  4   1946      5            1
  5   1946      4            0 #order is not important
  6   1947      1            0
  7   1947      2            0
  8   1947      3            1
  9   1947      4            1
  10  1947      5            0
  </code></pre>
  <p>I tried to use <code>complete()</code> from tidyr package, but it's not succeeding...</p>