Postal service specs based on examples in the readme
[work in progress]
. PostalService now inherits from Spree::ShippingCalculator
  . The commit-e09735c622326b2f2e14ee2c3de212145eb83894 on spree-core forces Shipping calculators to inherit from Spree::ShippingCalculator only. Otherwise tthe calculator doesn't show up in the available calculators list. [Ref: https://github.com/spree/spree/commit/e09735c622326b2f2e14ee2c3de212145eb83894]
  . There is also a corresponding change in directory structure and in engine.rb
. PostalService#compute changed: accepts a Stock::Package object as argument instead of Order or LineItems [Tests yet to be changed]
. PostalService#available? changed: accepts line_items as arguments instead of an Order [Tests yet to be written]
. PostalService available? and item_oversized? method arguments changed:
 . available? accepts an array of Stock::Package#contents
 . item_oversized? accepts a variant
. Logic change in PostalService#item_oversized?
 . The existing logic was incorrect. An Item is oversized if its longest and second-longest sides are greater than the set limits. The older logic used only the first value from an ascending-ordered array of sides. Now it sorts in descending order, and uses the first two values.
. Specs changed for PostalService#compute as the method argument changed in previous commit.
. Specs added for available? and item_oversized? methods

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