Require Shipping Calculators to Inherit from Spree::ShippingCalculator

[Fixes #3116]
Add #in_stock? method to Spree::Variant

As proposed by @jdutil in spree/spree_wishlist#37

[Fixes #3163]
[api] Return attachment URLs using the avaialble styles

This is so an API endpoint can render all the styles of an image, not just the Big One.
[api] Add CHANGELOG entries for products+images API changes
Cache product API responses for 3 minutes to make things like Backbone cache them
Add spot in spree application where JS templates can be yielded to

By default, this does nothing.