Khanload & topic tree update

If this looks OK, will merge to release-0.11.

This fixes the `khanload` scripts for 2 issues:
1. KA changed the underlying API; had to tweak a URL after chatting with them
2. The current topic tree is self-referential (both parents and children are kept); parents must be removed before trying to save.

Testing:
* Take a look at the new topic tree.  Is it OK?  Should we eliminate the grades?  Should we add a switch on which parts of the topic tree to use?

@wangguan59 @rtibbles your advice is requested!

![image](https://f.cloud.github.com/assets/4072455/1570770/d3f3da7e-50f4-11e3-8a8b-1f7947de613b.png)