Use stats objects instead of hashes

It will make it far easier to call other methods on the stats object,
and we're already caching the methods.

We had to remove the view fragment caching because the stats object
isn't as easy to cache. The good thing about it is the view will
automatically be updated when we change logic regarding which stats to
show, and the methods taking long to execute are cached in the model.

app/controllers/polls_controller.rb

@@ -33,7 +33,7 @@ class PollsController < ApplicationController
   end
 
   def stats
-    @stats = Poll::Stats.new(@poll).generate
+    @stats = Poll::Stats.new(@poll)
   end
 
   def results