Make bok-choy correctly publish units with children
Implement children for pure XBlocks in edx-platform

This requires fixing the javascript api implementation, and adding
an implementation of get_block to the ModuleSystem api.

However, the implementation is incomplete, due to mismatches between
the expectations of XModule and XBlock.

Also adds tests using the Acid block to make sure that the javascript
and python apis for children are working correctly.