Make XmlModuleStore handle asides

@cpennington this pulls in your changes; so, it needs to be rebased after you merge yours and is blocked by yours. 

@cpennington @doctoryes Just review https://github.com/edx/edx-platform/commit/38a3da81bb74aa97cfdd7181ddeca718ee6de895 which is the real PR
Could these parameters be documented in the docstring? Particularly, `system` will be bewildering to coders unfamiliar with this part of the code.