Use ActiveSupport::Cache::NullStore if ActionView::Base.cache_template_loading is disabled

This should take care of all the confusion people were having with
issues like rails/cache_digests#11, rails/cache_digests#13, and
rails/cache_digests#24.
Merge pull request #48 from shuber/dev-cache-store

Default to Cache::NullStore when ActionView::Base.cache_template_loading is disabled