title: Add time unit to Joda DateTime object
body: <p>Is there a better way to add a certain unit of time to a DateTime object?</p>
  
  <p>I would like to do something similar to:</p>
  
  <pre><code>TimeUnit unit = TimeUnit.valueOf("days");
  DateTime date = new DateTime().add(unit, 20);
  </code></pre>
  
  <p>This is my current implementation. </p>
  
  <pre><code>int number = 20;
  MutableDateTime now = new MutableDateTime();
  String timeUnit = getTimeUnit(); // returns "days", "months", "years"
  if (timeUnit.equals("days"))
      now.addDays(number);
  else if (timeUnit.equals("months"))
      now.addMonths(number);
  else
      now.addYears(number);
  return now.toDateTime();
  </code></pre>