Removing empty sub-topics, part I

This branch is dependent on #399 (for testing logic) and #400 (other khanload changes).
This branch addresses issue #275 , and enables addressing issue #353

Issue:
In the topic tree, if you click on some sub-topics, the next page shows nothing.

Current fixes:
* Because we don't support all khan exercises, we are forced to remove topics with no leaves.  If we remove topics, but don't delete all the metadata, we get this problem.
* In addition KA sends us some topics with no leaves.  These must be scrubbed as well.
* A test case for testing that all pages have at least one exercise was implemented.

Not fixed:
* Finally, htere are a number of topics that have exercises showing, but their position on the page is not proper, and so don't show.  This will need to be debugged to solve #353, and the test created here should be extended for that case.