MC-45: Connects to Magento, sends products and manage errors return

@jjanvier I can't spec this class because of SOAP call ;) But https://github.com/akeneo/MagentoConnectorBundle/blob/MC-45/spec/Pim/Bundle/MagentoConnectorBundle/Validator/Constraints/MagentoReachableValidatorSpec.php are specs I did for the class where I removed the SOAP call. It's an important validator, that's why I preferred create this MagentoSoapClient to be able to spec the validator.