Add warnings for Stripe obj returns, and change connection token endpoint (stripe#16)

As the warnings say, our example code creates a direct dependency between the server's
`Stripe.api_version` and the clients that consume the example backend responses.

This means clients must also be prepared for Backwards-compatible changes at any time
https://stripe.com/docs/upgrades#what-changes-does-stripe-consider-to-be-backwards-compatible

And it means that for the server to adopt a breaking change, all of their clients must 
also be prepared for it.

Changing the `/connection_token` so that it is *not* dependent on the Stripe response, 
and putting notes on the other two.

Author: danj-stripe

web.rb

@@ -71,6 +71,10 @@ def validateApiKey
   log_info("Reader registered: #{reader.id}")
 
   status 200
+  # Note that returning the Stripe reader object directly creates a dependency between your
+  # backend's Stripe.api_version and your clients, making future upgrades more complicated.
+  # All clients must also be ready for backwards-compatible changes at any time:
+  # https://stripe.com/docs/upgrades#what-changes-does-stripe-consider-to-be-backwards-compatible
   return reader.to_json
 end
 
@@ -95,7 +99,7 @@ def validateApiKey
 
   content_type :json
   status 200
-  token.to_json
+  return {:secret => token.secret}.to_json
 end
 
 # This endpoint creates a PaymentIntent.
@@ -106,7 +110,7 @@ def validateApiKey
     status 400
     return log_info(validationError)
   end
-  
+
   begin
     payment_intent = Stripe::PaymentIntent.create(
       :payment_method_types => ['card_present'],
@@ -179,5 +183,9 @@ def lookupOrCreateExampleCustomer
   log_info("Attached PaymentMethod to Customer: #{customer.id}")
 
   status 200
+  # Note that returning the Stripe payment_method object directly creates a dependency between your
+  # backend's Stripe.api_version and your clients, making future upgrades more complicated.
+  # All clients must also be ready for backwards-compatible changes at any time:
+  # https://stripe.com/docs/upgrades#what-changes-does-stripe-consider-to-be-backwards-compatible
   return payment_method.to_json
 end