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
Removed duplicate open/close adjustments methods.

Fixes #2947
Fix issue when adding products without option values

Variants are not removed along with its options values and they
shouldn't because they might be associated with one or more values and
they hold other important infos

When admin deletes an option type all variants associated with that
record are not removed. However they were not displayed in admin
anymore due to a condition which did not allow to display variants with
no option values. Variants were still displayed on frontend though,
or their radio buttons at least

This patch aims to:

  * allow admin to associate variants with new option values and edit
    option values associated with a variant. I couldn't see any reason
    why admin can't to that
  * Make better use of both Spree and Rails FormHelper API
  * Hide variants that does not have any option values associated on
    frontend product page
  * Fix #2276. It adds a new method to Product to check if it contains
    variants with values associated. Denpending on that result the
    params sent to order#populate changes and nothing is actually sent
    populated

Fixes #2937