Add support for calling the invoicing API

[Sthing]

No description provided.

[lsolesen]

Thanks for the pull request. Do you mind writing a test case scenario for the change, so we avoid problems in future releases?

[lsolesen]

@Sthing What about introducing a new class for the Invoicing API so the resulting end code will be more clear?

$client = new QuickPay_Invoice();

And that class utilize the client with the url?

[lsolesen]

@Sthing What is your take on my suggestion for introducing another class for starting up the Invoice API?

[Sthing]

@lsolesen Ehm - I think it is ok, but I don't have the time to do it right now.

[welrachid]

Not sure this is still relevant.
https://github.com/QuickPay/quickpay-php-client/pull/84/commits

i made this solution, so i dont have to deal with multiple classes, since the only difference is the api endpoint. The rest is the same and therefore no need for another class.