Added Napa.cache to wrap ActiveSupport::Cache

Makes it easy to access `ActiveSupport::Cache` the same way you would with `Rails.cache`, except with `Napa.cache`.

@danielmackey @jdoconnor 
yea, exactly, so it's just always there if you want to use the `:memory_store`, otherwise you can override it with any other store supported by `ActiveSupport`