title: Select data.frame rows from specific days of the week
body: <p>I have the following data.frame:</p>
  
  <pre><code>           Bounty Snickers Mars
  2014-10-02    400      200 1000
  2014-10-03    400      100  100
  2014-10-04    100      100  500
  2014-10-05    800      900  600
  2014-10-06    400      300  400
  2014-10-07   2000     1500  800
         ...    ...      ...  ...
  </code></pre>
  
  <p>Is there a way to select all rows that fall, for example, on a Monday, or on a weekend?</p>