title: Hive function: add month to date
body: <p>I'm searching for a function in Hive, that'll add month to date.</p>
  
  <p>For example :</p>
  
  <pre><code>add_month('2014-01-01',1) -&gt; '2014-02-01'
  add_month('2013-12-01',1) -&gt; '2014-01-01'
  </code></pre>