Add #cache_key method to models

[ryantownsend]

This method helps Rails with clean caching, e.g.

Controller:

def show
  @menu = ::FlexCommerce::Menu.find("main-menu")
  fresh_when(@menu)
end

View:

<% cache(@menu) do %>
  ...
<% end %>

It's defined on FlexCommerce::ApiBase which all the models inherit from, but only works if a given object has both an id and updated_at attribute, if either are missing a NotImplementedError will be raised and that model will have to manually define it's cache_key method.