Add a cache layer

[mrhappyasthma]

I need to determine a good TTL, but a cache layer might be nice for speed and to avoid external URL fetch costs.

Data updates, at most, once a quarter. So caching for week or maybe months at a time is probably pretty safe without losing much accuracy.

[mrhappyasthma]

The URL fetch quota on App Engine is more permissive than the new python3 datastore limits. So this may be infeasible while trying to ensure that I stay below the free budget thresholds using App Engine.

[mrhappyasthma]

App Engine now longer has free budgets for this... But maybe it's still worth the investment to limit queries out to our data fetcher (i.e. MSNMoney).

Things like the Stock IDs or even general stock data could probably be cached for a while.