Add support for Meteor 0.7.x.

 * Wrap the client interval in a startup function so that the interval
   is set up once `Meteor.settings` reaches the client.
 * Centralize defaults in `config.js`
 * Wrap server code in `Meteor.startup` so that it runs after user app
   has a chance to configure the settings.
 * Add instructions for setting up configuration in `README.md`