[UPDATE] enhancements and refactoring

Closes #25.

Simplification of the instantiation of Service objects in the
generated sample file by removing the SoapClient options parameter. By
default, the cache_wsdl is disabled, the wsdl_url is set with the WSDL
url used to generate the package and trace is set with true. The
VALUE_WSDL_URL constant has been added to the generated WsdlClass
defined with the WSDL url used to generate the package.

Remove trailing ?> in generator files/classes.

Add WsdlToPhpGenerator::getWsdl($_index) method to get any WSDL at any
position.

Enhance generated sample file with explanation about the optional
parameter allowing to override the SoapClient Options. Remove the $wsdl
parareter previously used to instantiate each Service object.

Avoid Struct object to reset SoapClient when it is instantiated when
calling the parent constructor.