fix for report locale names when extending via class_eval

* source issue: https://github.com/spree/spree/issues/4086
* try to maintain the reports not in a constant
* make it easier to add new reports over current merging of hash of hashes way

Example decorator now becomes:

https://gist.github.com/HoyaBoya/7890846

As opposed to what people are doing here for example:

https://github.com/jenius/spree_advanced_reporting/blob/master/app/controllers/spree/admin/reports_controller_decorator.rb#L5-L11