title: pandas groupby concatenate strings in multiple columns
body: <p>I have this pandas data frame:</p>
  
  <pre><code>df = DataFrame({'id':['a','b','b','b','c','c'], 'category':['z','z','x','y','y','y'], 'category2':['1','2','2','2','1','2']})
  </code></pre>
  
  <p>which looks like:</p>
  
  <pre><code>  category category2 id
  0        z         1  a
  1        z         2  b
  2        x         2  b
  3        y         2  b
  4        y         1  c
  5        y         2  c
  </code></pre>
  
  <p>What i'd like to do is to groupby id and return the other two columns as a concatenation of unique strings.</p>
  
  <p>The outcome would look like:</p>
  
  <pre><code>  category category2 id
  0        z         1  a
  1      zxy         2  b
  2        y        12  c
  </code></pre>