Add support for Meteor 0.7.x.

Heya! I wanted to use this in our project, but it took a bit of work to get it working properly with Meteor 0.7.0.1. The biggest problem was that the heartbeat interval would try to hook itself up before the client had received `Meteor.settings`. 

As a bit of a bonus I centralized the config defaults into `config.js`, but that required a bit more refactoring (notably wrapping things in `Meteor.startup` to give the user apps a chance to configure things first).

Criticisms welcome!

Here's the summary:
 * 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`