Implement XML input/output for AssetMetadata.

@cpennington @dmitchell 
Note that the tests use an XSD to validate the generated XML. The XSD is kept in the test directory for now - but a better place should be found in the future.
Is `dateTime` == python's iso format: that is, is it string w/ timezone encoded as `(+|-)HH:MM`? Python's iso format is, afaict, not ISO (it encodes the timezone in a manner which javascript and mongo can't handle).