Support nested phrase objects

* Support both in `extend()` and in the constructor.
* Update docs & README.
* Update version to 0.4.0.

```js
polyglot.extend({
  "nav": {
    "hello": "Hello",
    "hello_name": "Hello, %{name}",
    "sidebar": {
      "welcome": "Welcome, %{name}"
    }
  }
});

polyglot.t("nav.sidebar.welcome", {name: "Raph"});
=> "Welcome, Raph"
```

Closes ,https://github.com/airbnb/polyglot.js/issues/10, https://github.com/airbnb/polyglot.js/pull/11, and maybe https://github.com/airbnb/polyglot.js/pull/8.