Simplify API.

Change API to one universal 'create' method which requires the notification type ('success', 'info', 'warning', 'error') as the first argument.

This is useful if you use JSON responses from a server to handle notifications dynamically, e.g. the data received from the server includes a status object with the notification type, which is then passed to the create method like so:

```javascript
var callback = {
  status: 'success',
  message: 'This request was pretty successful.',
  title: 'Titles are always sexy'
}

toastr.create(callback.status, callback.message, callback.title);
```