Set a default date

Hi!

Maybe just modifying this line we can set a default date:

https://github.com/Meshredded/react-simple-month-picker/blob/master/src/component/month-picker.js#L14

```js
this.state = {cells:[], selectedDate: props.selectedDate || new Date(0), currentView:"years",renderDate:false, months:months};
```

```js
selectedDate: props.selectedDate || new Date(0)
```