EAD import/export mapping updates

Implements a variety of mapping changes, especially in terms of attributes, tag sequences, etc. With these changes, I can establish that valid EAD instances will be exported in most cases. 

It's probably worth specifically noting the changes in 8f21ab368cfafa2389cda2ff5ba6adc11c08f2fe - they deal with how the serializer handles content containing XML. It's not elegant, and the tests probably need to be a bit more robust in particular. I am operating under the presumption that the content within those properties is well-formed.