Proposing the Coordinator functionality

If one needs to react on two (ore more) asynchronous fired events the coordinator tracks them and triggers a callback.

```
var dependency1 = new Dependency1();
var dependency2 = new Dependency2();

var coordinated = async.coordinator( ['dep1', 'dep2'], function() {
    console.log('all dependencies are initialized.');
});

dependency1.on('initialize', function() {
    coordinated('dep1');
});

dependency2.on('initialize', function() {
    coordinated('dep2');
});
```