Merge shipping, tax, and discounts submodules into core

[jedateach]

Development on the discount and shipping modules has progressed well. They are now getting more co-dependent. I thin it is time to commit to these features and combine them into the core shop module.

Currently the issues appear to be arising with the modifiers system. I'm thinking it might be time to do away with order modifiers: #128

[wildflower]

I haven't read any of the code so this may be irrelevant but, does the shipping modules have different types of Track and trace links for different couriers? NZ Post, FedEx, DHL etc it think they all do it slightly differently

[jedateach]

It doesn't currently, but I've tried to abstract with that in mind.

The ShippingMethod base class has a calculateRate function to be overridden, where you can do API fetching etc.

Here's some code to demonstrate:

class NZPostShippingMethod extends ShippingMethod{

    function calculateRate(ShippingPackage $package, Address $address){
        //fetch rate via API ...
    }

}

[wildflower]

Track and Trace would be more at the end of the Order process when the store owner puts the item in a box and 'Ships' the order.
I guess it could effect email templates, or the back end screens, it could be a standard field but each courier would have their own URI format

[jedateach]

Oh, sorry I didn't read your comment properly. No track & trace built in yet.

Might be worth borrowing ideas from https://github.com/Shopify/active_shipping

[wilr]

Archiving as separate modules was decided the way to go.