Support additional headers in requests

[lauer]

This is specially nice, when a special CallBack url is needed, when testing.

A solution was started in #17, but never merged and completed.

Pull request will follow

[AndreasHerss]

Can't believe this isn't supported... :(

[Sthing]

Thank you for the PR. Included in https://github.com/QuickPay/quickpay-php-client/releases/tag/1.1.0