Add some way to control order of created nodes

[rlskoeser]

There is some preliminary work in #14 for controlling order of nodes created when terms are passed into the constructor, but supporting order there without supporting it more generally seems strange.

I don't think it requires a subclass, should be optional behavior on default XmlObject class.

We'll have to figure out a way to specify order on the class, and then figure out how to access that logic on the field descriptors so node creation can be constrained by the configured order.