2.0.0.0-dev56

* Fixed bugs:
  * Fixed placing order with PayPal Payments Advanced and Payflow Link
  * Fixed losing previously assigned categories after saving the product with changed category selector field
  * Fixed losing of a newly created category assignment after variations generation during Configurable product or Gift Card creation
  * Fixed the error in order placement with Recurring profile payment
* GitHub requests:
  * [#299](https://github.com/magento/magento2/pull/299) -- Fix for issue Refactor Mage_Rating_Model_Resource_Rating_Collection
  * [#341](https://github.com/magento/magento2/pull/341) -- Replacing simple preg calls with less expensive alternates
* Modularity improvements:
  * Layout page type config moved to library
  * Design loader moved to library
  * Theme label moved to library
* Themes update:
  * Reduced amount of templates and layouts in magento_plushe theme
  * Responsive design improvements
* Integrity improvements:
  * Covered all Magento classes with argument sequence validator
  * Added arguments type duplication validator
* Implemented API Integration UX flows:
  * Ability to create and edit API Integrations
  * Ability to delete API integrations that were not created using configuration files
* Removed System REST menu item and all associated UX flows:
  * Users, Roles, and Webhook Subscriptions sub-menu items were removed
* Removed the Webhook module