Document newer config loading in SDK

[edjiang]

var client = new stormpath.Client({
  apiKey: {
    id: process.env['STORMPATH_CLIENT_APIKEY_ID'],
    secret: process.env['STORMPATH_CLIENT_APIKEY_SECRET']
  }
});

Quickstart says to:

var apiKey = new stormpath.ApiKey(
  process.env['STORMPATH_CLIENT_APIKEY_ID'],
  process.env['STORMPATH_CLIENT_APIKEY_SECRET']
);

var client = new stormpath.Client({ apiKey: apiKey });

Which is way less convenient.

(Alternatively, shouldn't they be loaded from environment variables by default, anyways? So no need to even specify this)