added analytics tracking code w/ per-env config

I've added a config variable (via config/environments) to specify a snippet of code to include if you want web traffic analytics.  The analytics provider we've chosen for our production site is http://clicky.com, which has good privacy protections for Growstuff's visitors -- in particular, we have set it up so that it drops the last octet of any IP address, so that they aren't storing personally identifying information about our visitors.

Their specific terminology wrt the IP address privacy option is: "Enabling this will replace the last octet of all IP addresses with a "0". If users from the same subnet visit your site at the same time, they will still be counted as separate unique visitors, as long as they have cookies enabled."