Code refactroing

[Daniel217D]

• [x] Set up composer
• [x] Improve assets generation via gulp
• [x] Add a script to archive the plugin for further installation
• [x] Add wpcs (WordPress Code Standarts) to check code quality
• [x] Remove commented out or simply unused code
• [x] Add function to replace wc_get_template in plugin
• [x] Remove code duplication in gateways
• [x] Set up namespaces and code autoloading
• [x] Improve code splitting into logical parts

[cpiechotta]

With regards to this: "Move settings from the Reepay Checkout gateway to a separate settings page"

Please leave all Reepay settings under Payments -> Reepay. Please be a good WooCommerce ecosystem citizen and do not deviate from the conventions.