Error in README example

From README:

```ruby
# Create a button with a custom bg & image
    swipe_btn = UIButton.custom
    swipe_btn.setBackgroundImage("nav_bar_menu_show_bg".uiimage, forState: :normal.uicontrolstate)
    swipe_btn.setImage("nav_bar_menu_show".uiimage, forState: :normal.uicontrolstate)
    size = "nav_bar_menu_show_bg".uiimage.size
    swipe_btn.frame = CGRectMake(0, 0, size.width, size.height)
```

`UIButton.custom` gives `undefined method custom' for UIButton:Class (NoMethodError)`. Not sure what the correct alternative is?