Added code example for setenv class
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
gerhardsam/puppetlabs-apache,gerhardsam/puppetlabs-apache,bmjen/puppetlabs-apache,buzzdeee/puppetlabs-apache,yakatz/puppetlabs-apache,traylenator/puppetlabs-apache,puppetlabs/puppetlabs-apache,digitalmediacenter/puppetlabs-apache,buzzdeee/puppetlabs-apache,svanhart/puppetlabs-apache,puppetlabs/puppetlabs-apache,dzmitryv/puppetlabs-apache,dzmitryv/puppetlabs-apache,samuelb/puppetlabs-apache,FlatKey/puppetlabs-apache,digitalmediacenter/puppetlabs-apache,guegue/puppetlabs-apache,eputnam/puppetlabs-apache,svanhart/puppetlabs-apache,eputnam/puppetlabs-apache,yakatz/puppetlabs-apache,bmjen/puppetlabs-apache,guegue/puppetlabs-apache,FlatKey/puppetlabs-apache,traylenator/puppetlabs-apache,samuelb/puppetlabs-apache