Coupon - Refactor the way conditions are managed when building Coupon

The way I initially developed the Condition module gave a good genericity but lacked of extensibility.

Basically you couldn't create a complex Condition 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/26

Less crappy unmaintainable javascript
More logic in extendable php
I simplified the code, so it should be more maintainable.

Cheers