ensure indexes on the date, type, and sgv fields at startup

@bewest let me know what you think, I'd like to get this into the 0.5.0 release since Chris from mongolab said some user's sandboxes are now effecting others.

This turned out to be a bigger change than I expected since it needed to wait for the mongo connection to be ready.  I tested that it works locally, but not with production data yet.
It's nice to export a function, but I'm not going to argue with this... were you planning on exporting `{ ... }` everywhere?
EG, express almost always exports a function I think.