Get detailed/lifetime stats in machine-consumable format

This is a split from issue #127, which was actually two issues.

Right now you can get detailed stats, but only in a human-readable string form. So the feature that will be added to to return those stats in JSON.

Why JSON? Because adding a struct adds a commitment to support that exact ABI in Steamworks in perpetuity, which is a relatively large maintenance burden; I think out of proportion for this feature. JSON is a reasonable choice because things can be added/removed without changing the ABI. We'll need to make sure that people do not build critical functionality on top of it, and only use it for analytics, so that if fields to be removed or renamed, it will only break the analytics, and not any more important user functionality.