Mollie API also allows a webhookUrl. The PurchaseRequest now accepts
this parameter.

For more info:
https://www.mollie.nl/files/documentatie/payments-api.html
composer.json was modified by my IDE, reverted
code conventions: PSR2
webhookUrl -> notifyUrl
removed methods, they are already available in AbstractRequest
php cs fixer: removed white space
Merge pull request #8 from frederikbosch/master

Allowance for webhookUrl parameter.