title: react-datepicker with no initial value
body: <p>I started using the react-datepicker component.
  <a href="https://github.com/Hacker0x01/react-datepicker" rel="noreferrer">https://github.com/Hacker0x01/react-datepicker</a></p>
  
  <p>I got the example running and now I want to adapt it that there is no initial value.
  The example code is the following:</p>
  
  <pre><code> &lt;script type="text/javascript"&gt;
   var container = document.querySelector('#datepicker-container');
   var exampleComponent = React.createClass({
      displayName: 'exampleComponent',
      getInitialState: function() {
        return {
           start_date: moment(),
        };
      },
      handleStartDateChange: function(date) {
         this.setState({
            start_date: date
         });
      },
     render: function() {
        return React.DOM.div({}, [
           DatePicker({
           key: 'example1',
           selected: this.state.start_date,
           onChange: this.handleStartDateChange
        }),
  
        ]);
     }
  })
  React.renderComponent(exampleComponent(), container);
  &lt;/script&gt;
  </code></pre>
  
  <p>I tried to use <code>selected: none</code> or even leave <code>selected</code> out but then I get the following error:</p>
  
  <pre><code> TypeError: newProps.date is null
       value: newProps.date.format(this.props.dateFormat)
  </code></pre>
  
  <p>I also looked into the source code, but didn't find any possibility to start with an empty date.
  Thanks in advance for your help!</p>