Extend VatCode resource

[iranl]

Adds get(), send() and Mapper to VatCode.

Includes fixes to listAll() code to:

1. Properly process options passed to listAll in the following format:

$options['vattype'] = 'sales';
$vatcodeApiConnector = new \PhpTwinfield\ApiConnectors\VatCodeApiConnector($connection);
$vatcodes = $vatcodeApiConnector->listAll('*', 0, 1, 100, $options);

2. Handle situations where only one result is returned from listAll() and Twinfield uses a different format when returning results

[willemstuursma]

Thanks, it could use some tests but it looks fine.

That's a lot of pull requests you are contributing! Working on a Twinfield related project?

[iranl]

Thanks, it could use some tests but it looks fine.

That's a lot of pull requests you are contributing! Working on a Twinfield related project?

Been using the Twinfield API in about 5 projects using my own horrendous code since 2014 based on the PHP Example by Twinfield. But in my own defence there wasn't a lot of documentation or libraries like this one then.

The deprecation of session login and switch to OpenID Authentication seemed like a good moment to replace my own code with this library, but the library lacks some functionality I currently use (like listAll() for articles and invoices and invoice VAT lines), so thats why I'm creating all these PR's.

[willemstuursma]

Thanks @iranl, nice to hear that you are liking the library. Looking forward to merging all the changes.

[iranl]

Superseded by #142