Allow user to specify a UIBarButtonItem as the title for a nav bar button item.

Hey!

I needed a way to pass `self.editButtonItem` into the nav bar via `set_nav_bar_left_button` and came up with the following solution:

```ruby
set_nav_bar_left_button self.editButtonItem
# or
set_nav_bar_right_button self.editButtonItem
```

Thanks!