Add FeatureDetection.hardwareConcurrency

Chrome Canary recently added `navigator.hardwareConcurrency` which exposes the number of physical cores on the client.  This is a simple change to expose that value (or a default if not defined) on `FeatureDetection`.

I chose 3 as the fallback based on [Steam's hardware survey](http://store.steampowered.com/hwsurvey/), which shows 2 cpus 47.26% of the time and 4 cpus 44.94%.

This will be handy when we do #1487 (probably post 1.0), but is useful immediately for `Primitive` creation.

On a related note, is `FeatureDetection` going to be public or private, because if it's public, it's missing a bunch of documentation.