Order::fromXml

WIP Adding static method to build Order from a SimpleXMLElement object.

This makes it easy to building an Order from an Xml input.

I have some questions about the target order model.

1) I am unclear what the Meta fields are for.

2) It seems to me that the shipping address should be stored at the order/shipment level, rather than at the line item level.
Merchantry's xml has the shipping address at the order level and that is also the level we use in OpenSky's order model.