FriendRequest is not export to npm module 

i'am trying v0.4.0 with npm install

how to get a FriendRequest instance while using `require('wechaty')` ?

``` javascript
const {Wechaty,Room,Contact,FriendRequest} = require('wechaty')
```

`FriendRequest` is undefined

i'am testing the following code.

``` javascript
const sender : Contact = message.get('from')
const fr = new FriendRequest()
fr.send(sender, 'hello~')
```
