title: R convert a data.frame to a daily time series object
body: <p>I have my data.frame some what like this</p>
  
  <pre><code>  name units_sold order_date
  1 obj1         10 2013-09-21
  2 obj1         10 2013-09-22
  3 obj1         10 2013-09-23
  4 obj2        100 2013-09-21
  5 obj2        200 2013-09-22
  6 obj2        300 2013-09-23
  7 obj3         70 2013-09-21
  8 obj3        200 2013-09-22
  9 obj3         50 2013-09-23
  </code></pre>
  
  <p>I want to convert it to a time series object such that it should have values in below format:  </p>
  
  <pre><code>       2013-09-21  2013-09-22 2013-09-23
  obj1      10            10         10
  obj2      100           200        300
  obj3      70            200        50
  </code></pre>
  
  <p>... for a week</p>