Respect WSDL type definitions when decoding and support classmap option

[clue]

The Client now respects the WSDL type definition when decoding and supports the classmap option.

You can use the classmap option to map certain WSDL types to PHP classes
like this:

$client = new Client($browser, $wsdl, array(
    'classmap' => array(
        'getBankResponseType' => BankResponse::class
    )
));

This is a BC break because the decoder did previously only use the WSDL type definitions for encoding the outgoing SOAP request messages, but not for decoding the incoming SOAP response messages.

Builds on top of #32