Aside import/export

PLAT-220

@cpennington @doctoryes plz review. I need to fix the pre-existing error which I marked as FIXME using the 4 stage id lookup we discussed at standup, but as far as xml serialization/deserialization goes, this is done.
I'm curious why you went this route, rather than asking the aside to serialize and then checking the output. Alternately, maybe we should have a method on Asides (with this as a default) so that we can ask them if they have any XML to serialize? I'm worried a bit about the possibility of an aside that is storing data not in an XBlock field, but wants to serialize that data to XML.