[UPDATE] minor fix and improvements

Fix generic WsdlClass::__set_state() method in order to instantiate
dynamically the object without any parameters.

Improve "from schema" meta storage by providing the constant
WsdlToPhpModel::META_FROM_SCHEMA and two methods
WsdlToPhpModel::setFromSchema() and WsdlToPhpModel::getFromSchema().
These two methods are then used within WsdlToPhpGenerator methods to
store this information