[bundle] Rename api key to secret key
# Please enter the commit message for your changes. Lines starting
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#	modified:   Client/Authentication/AuthenticationStrategyInterface.php
#	modified:   Client/Client.php
#	modified:   Controller/DemoController.php
#	modified:   DependencyInjection/Configuration.php
#	modified:   DependencyInjection/JACPaymentStripeExtension.php
#	modified:   Entity/DemoOrder.php
#	modified:   Form/StripeType.php
#	modified:   JACPaymentStripeBundle.php
#	modified:   Plugin/Exception/AuthenticationException.php
#	modified:   Plugin/StripePlugin.php
#	modified:   README.md
#	modified:   Twig/PaymentStripeExtension.php
#
[bundle] Fixes and additions to code comments