Updated topics.py to correctly create the nodecache from latest KA API.

Updated nodecache and topics.py.

Updated Video navigation for new keys.
Bug fix: Khan Academy had removed related videos from exercises data.

Now retrieved via separate API call.
Updated topics.py to correctly download all topic data.

Also now generate knowledgemap data, and download icons.

Missing icons are replaced by the default icon.
Script to pull latest Khan-Exercises repository.

KA Lite platform is not currently compatible with latest exercises.
Remove knowledgemap utils - this functionality is now within topics.py
Merge branch 'develop' into topicfixing

Conflicts:
	kalite/templates/topic.html
	kalite/utils/topics.py
updated scripts for pruning unknown exercises.
fixing small bug made apparent by exercise-eliminating.
Merge remote-tracking branch 'upstream/develop' into topicfixing
documenting code more carefully, splitting up functions more clearly, combining files.
updates to script to download icons, refresh for missing icon
remove improperly downloaded badge.  Change all refs of settings.LOG to logging
renaming to get_topic_tree; extending get_node_cache funtion
Merge pull request #211 from bcipolli/topicfixing

Re-enabling topic downloads from Khan Academy