[COOK-3290] Adding correct EPEL urls & keys for Fedora: 18, 19

Note: This fixes [COOK-3290](http://tickets.opscode.com/browse/COOK-3290)
      for the `yum::epel` repo, and adds partial support for fedora to
      this cookbook.  I didn't test the `yum::ius`, `yum::repoforge`,
      and `yum::yum` recipes on fedora.  It was not clear to me whether
      these repos officially supported or would work on fedora 18 & 19,
      and as I didn't have enough time to check this, they may not work fully
      on fedora.