Piwik: added settings.yml integration

Rationale:

 * In production mode, analytics MUST be configured, via the following
   in your config/settings.yml:

     :piwik:
       :url: "https://backup1.panmind.org/piwik"
       🆔  1

   If you *really* want to disable analytics in production mode, there's
   a specialized setting for that:

     :piwik:
       :i_really_want_to_disable_analytics: true

   Useful on pmdev.* and on developer working copies running in production
   mode for testing purposes.

 * In development mode, analytics tags aren't generated, so no config
   is needed.

 * In test mode, a dumb "test.host/piwik" configuration is generated
   automatically.

The settings.yml.sample file has been updated with relevant settings.

lib/pm/piwik.rb

@@ -1,24 +1,56 @@
 module PM
   module Piwik
-    def piwik_base
-      #request.protocol + 'backup1.panmind.org/piwik/'
-      'https://backup1.panmind.org/piwik/'
+    if Rails.env.production?
+      Config = APPLICATION_CONFIG[:piwik]
+
+      # *Enforce* analytics configuration in production mode
+      #
+      # If you're developing and want to run your local copy
+      # in production mode, you can either add dumb settings
+      # in your settings.yml, to check how the Piwik JS code 
+      # is being generated:
+      #
+      #   :piwik:
+      #     :url: http://localhost/piwik
+      #     :id:  31337
+      #
+      # Or, if you're really not interested at all in any of
+      # the Piwik bells and whistles, you can boldly disable
+      # it via the following configuration:
+      #
+      #   :piwik:
+      #     :i_really_want_to_disable_analytics: true
+      #
+      raise ArgumentError, 'Analytics configuration missing' if Config.blank?
+    elsif Rails.env.test?
+      Config = {:url => 'http://test.host/piwik', :id => 420}
+    end
+
+    def piwik_url
+      #request.protocol + APPLICATION_CONFIG[:piwik][:path]
+      Config[:url]
     end
 
     def piwik_id
-      1
+      Config[:id]
     end
 
     def piwik_js
-      piwik_base + 'piwik.js'
+      piwik_url + '/piwik.js'
     end
 
     def piwik_php
-      piwik_base + 'piwik.php'
+      piwik_url + '/piwik.php'
+    end
+
+    def piwik_disabled?
+      Rails.env.development? || Config[:i_really_want_to_disable_analytics]
     end
 
     Piwik = ''
     def piwik_analytics_tags
+      return if piwik_disabled?
+
       if Piwik.blank?
         Piwik.replace(
           javascript_include_tag(piwik_js) +