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
I :+1: for renaming. "amount" is too generic, we need more explicit variable name.
Also, I prefer private properties and public accessors but I will accept your point as you are former of PaymentSuite :)