General Refactoring

This is a working PaymentSuite package implementing Paypal web Checkout.
Some improvements were necessary in order to make it compliant with other PaymentSuite payment methods.

Changeset:
 
* Added a payment_cancel route
* Renamed manager methods to be more explicit
* Removed unused class properties. Fixed some routes
* Adapting payment services
* Improved PaypalWebCheckoutMethod
* Passed php-cs-fixer and php-formatter
* Small style fixes
* Added URLFactory service
* Improved transaction check and PaypalWebCheckoutMethod
There is really little to do here, this is just the payment notification message coming from paypal. It's basically a *value object* representing the transaction. Actually even setters should be removed. Passing an `array` as a constructor is not an option as is almost always a smell.
The other option is to create some parameter factory service.