Support tint_color on toolbar buttons

Complete with passing specs! <3

example usage:

```ruby
    set_toolbar_items [{
      system_item: :flexible_space
    },{
      title: "Calculate Best Deal",
      action: :start_calculations,
      tint_color: UIColor.whiteColor
    },{
      system_item: :flexible_space
    }]
    self.navigationController.toolbar.barTintColor = rmq.color.purple
```

creates this:

![screen shot 2014-07-11 at 8 07 18 pm](https://cloud.githubusercontent.com/assets/139261/3560264/e71816d6-0960-11e4-8cf0-4bf408e8a376.png)