Added trace option

The methods __getLastRequest, __getLastRequestHeaders,
__getLastResponse and __getLastResponseHeaders only work if the
SoapClient object was created with the trace option set to TRUE.
Usage:
SoapWrapper::add(function ($service) {

$service->name('currency')->trace(TRUE)->wsdl('http://currencyconverter.
kowabunga.net/converter.asmx?WSDL');
});