Make XModuleDescriptor use the XBlock children API

N.B. When we are in a world of mixed XModules and XBlocks, the
system/runtime will have to be cognizant of that when asked to return a
XModule from an XModuleDescriptor child

[#LMS-190]
Merge pull request #462 from edx/cale/use-xblock-children-api

Make XModuleDescriptor use the XBlock children API