Feature: toolbars

This branch adds in the nice things talked about in #161.

Essentially, I extracted some code from the `set_nav_bar_button` method to reuse the same exact hashes to construct toolbar items. You then set them with `set_toolbar_items(buttons = [], animated = true)`. The `animated` parameter is for if the toolbar is hidden, setting the toolbar items will always show the toolbar so the user needs to choose if it's animated or not.

I also added the ability to use a custom view as a nav bar button or toolbar button.

Let me know what you think!