Use memcache to store temporary OpenID data

OpenID nonces and associations need to be visible across all the
servers in the production cluster, so store them in memcache where
everything can see them properly.

config/initializers/openid.rb

@@ -1 +1,7 @@
-OpenIdAuthentication.store = :file
+if defined?(MEMCACHE_SERVERS)
+  require "openid/store/memcache"
+
+  OpenIdAuthentication.store = OpenID::Store::Memcache.new(MemCache.new(:namespace => "rails", :string_return_types => true))
+else
+  OpenIdAuthentication.store = :file
+end