title: Need create column with type TIMESTAMP but does not update existing rows
body: <p>I have situation like this:</p>
  
  <ol>
  <li>have existing table</li>
  <li>need add new column using </li>
  <li>type of the column is TIMESTAMP</li>
  </ol>
  
  <p>Code:</p>
  
  <pre><code>&lt;changeSet author="name" id="bla-bla-bla"&gt;
      &lt;addColumn tableName="table_name"&gt;
          &lt;column name="new_col_name" type="TIMESTAMP"/&gt;
      &lt;/addColumn&gt;
  &lt;/changeSet&gt;
  </code></pre>
  
  <p>This code creating column, and its cool! But! Its also sets to all existing rows default value to 0000-00-00 00:00:00.</p>
  
  <p>But I need leave all existing rows without changes. And TIMESTAMPS should be set only for new rows.</p>