Coupon - Refactor the way parameters are stored

The way I initially developed the Coupon module gave a good genericity but lacked of extensibility.
Basically you couldn't create a complex Coupon like a "Coupon giving a product" or a "Coupon giving X percent only on a category" in a clean way. And you couldn't customize the way extra parameters were set (input, select, drag & drop, radio, checkbox, etc..).

This is now possible and documented on https://github.com/thelia/thelia.github.io/pull/25.

I simplified the code, so it should be more maintainable.
The Coupon table has been modified, you will need to rebuild the model.


I messed up with the file permissions, but set them back again.
And I accidentally added a submodule and then removed it.

Cheers