Adding Situation methods `caused` and `because`.

I'm hoping these will make using the API a little simpler. All they do is allow
you to pass a situation to another situation as either a cause using
`situation.because` or as an effect using `situaiton.caused`. eg:

```javascript
var slept_in = new Situation('5678');
var missed_work = new Situation('1234');
var got_fired = new Situation('0987');

got_fired.because(missed_work, console.log);

// or

slept_in.caused(missed_work, console.log);
```

The first parameters can be any of the following:

- An instance of Situation (with `.id`)
- a Situation's document body (with `._id`)
- a Situation's `id` (a string)
Updating Readme for new situaiton methods.