Expose app.env before app instance creation

It's not currently possible to pass environment-dependent options to the `EmberApp` constructor in your Brocfile without duplicating the environment-detection logic, because the canonical environment detection happens after the instance is created.

This change exposes a "class method" instead, so you can say things like:

````js
new EmberApp({
  someOption: EmberApp.env() === 'production' ? 'foo' : 'bar';
});
````