Added code example for setenv class

Example usage is based on the Apache docs' own example for the SetEnv directive: 
http://httpd.apache.org/docs/2.2/mod/mod_env.html#setenv

Change requested here:
https://tickets.puppetlabs.com/browse/DOCUMENT-204
Corrected style errors

Thanks for the edits, Hunter! Can you validate my corrections? I'm making these in-browser, and GitHub's editor might be throwing me off by converting my spaces to tabs.
Merge pull request #973 from psoloway/add-example-for-setenv-class

Added code example for setenv class