Add May and July 2013 Derby copy.

To test May copy:

* Change *DEMOS_DEVDERBY_CURRENT_CHALLENGE_TAG *  to *challenge:2013:may*
* Visit Dev Derby and read long ("Current") copy

To test July copy:

* Change *DEMOS_DEVDERBY_CURRENT_CHALLENGE_TAG *  to *challenge:2013:july*
* Visit Dev Derby, hover over July trophy, and read copy. Also make sure the tagline below the July trophy is showing up fine.