Performance changes 

I checked in code changes to enable caching, of crawling the site to pre-generate the cache, and a method for invalidating the cache.  Next is to call that cache invalidation function at the appropriate places (videodownload and videocheck commands, among others?)

This is in my "performance" branch.  It's not perfect (for example, I've checked in local_settings.py and put caching options in there), but I've submitted a pull request against learningequality/develop, so you can see the differences.