Handle 503 'Service Temporaily Unavailable' errors
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
Merge pull request #7 from ruudk/fix-500-error-handling

Handle 503 'Service Temporaily Unavailable' errors
Clarify transactionReference parameter

You can't pass in an 'id' param; it uses transactionReference
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.
A little typo...
Fix tests failing tests of pull request #10
Merge pull request #11 from Fuhrmann/patch-1

A little typo...
Merge pull request #10 from barryvdh/patch-2

Clarify transactionReference parameter
Merge pull request #12 from Fuhrmann/patch-2

Fix tests failing tests of pull request #10