first pass implementing 'paths' for video and exercise objects in node_cache.
Updates to make topic updates more robust.
Merge remote-tracking branch 'upstream/develop' into multipath_node_cache
add comments, rename functions to be more readable.
Merge pull request #215 from bcipolli/multipath_node_cache

Multipath NODE_CACHE
revamping the authorization decorators to generate Exceptions, then to define a Http403 handler to fail (on API) or redirect to login (on regular request)
Merge remote-tracking branch 'upstream/develop' into control_panel

Conflicts:
	kalite/main/views.py