Create endpoint for promotion adjuster in OrderUpdater

[mamhoff]

The code to handle promotions is scattered across four different classes:

• Spree::OrderAdjuster
• Spree::OrderContents
• Spree::Promotion
• Spree::Adjustment (see the #recalculate method)

Move all the code related to applying a promotion including eligibility checks to a new configurable class, Spree::Promotion::OrderAdjuster.

[jarednorman]

You have a lot of ideas today.

This (like the others) seems like a good idea.