Remove duplicate dependency for kaminari

Fixes #2944
Add short explanation to tax rate amounts to ease confusion
[frontend] Add another stub of spree_api_key in products_controller_spec to fix failing build

This build was broken: http://ci.spreecommerce.com/viewLog.html?buildId=6428&buildTypeId=bt7&tab=buildLog
Greatly reduce number of drawn routes on Spree::Core::Engine

By prepending routes as we were doing before, Rails was not checking to see if the routes already existed and therefore drew over 1,000 routes for Spree and its things. This, of course, would take time and RAM.

By calling draw, Rails checks and therefore doesn't draw the routes if they're already there.

Thanks to @parndt for pointing this out.
Remove old comment from OrderPopulator

Fixes #2939
[api] Remove LineItemDecorator

This is because variant_id and quantity are already attr_accessible on LineItems
Enforce usage of in screen.css.scss and add a new variable to the variables.css.scss file to allow for an easier customization

Fixes #2943
Extract taxonomy jstree menu to a new file