"Pull"/Receive Notifications

Implement the [Observer Pattern](http://en.wikipedia.org/wiki/Observer_pattern) such that modules can observe notification events that are **received from a user**, such as Twilio SMS or Email.

For example:

``` javascript
var NotificationCenter = require("./notification-center");

// Listen for an Event, received SMS
NotificationCenter.on("sms", function(event) {
// Event occurred
// Process SMS (Text message)
// Potentially reply
NotificationCenter.send(event.sender, ....);
});
// Listen for an Event, received Email
NotificationCenter.on("email", function(event) {
// Event occurred
// Process Email
// Potentially reply
NotificationCenter.send(event.sender, ....);
});

```

Related to #10.