Added a couple sanity checks to the config during the loadConfig call.
- Check that the config exists for the current NODE_ENV
- Check that the config.url exists and is valid structure
- Check that the config.database exists
- Check that the config.server exists and has a port and host value
Extracts all express-server-related code in index.js to core/server.js, leaving index.js purely for booting up Ghost's core components in a sensible order.
Aside from the project's tidiness, this means that we can perform asynchronous configuration loading/checks before requiring any modules that read the config.
