R16: Updates to extension and annotation (and essenceTrack annotation) structure.

[pdpinch]

Extension currently contains three discrete values (authority, type and value) within two fields. Also the extension can only contain key-value pairs and not hierarchical structures. Add third field extension and allow support for inline XML. Also since annotation is the extension of the instantiation, follow the same structure here as well.

[pdpinch]

with 5d0e20e, broke extension into 3 elements: extensionType, extensionValue and extensionAuthorityUsed.

[jackbrighton]

I'd suggest doing this also for the annotation, because many people are interested in recording things like PREMIS metadata for a specific instantiation.