[DRAFT] Rails picture tags

I'd like to spend a little more time getting the javascript side working better, but wanted some feedback first. I hope it's in the spirit of `<picture>`, and feels a little more like a decent rails/progressive enhancement approach.

```ruby
picture_tag 'page1.jpg, 'page2.jpg', 'page3.jpg'
```

```html
<picture>
  <source src="page1.jpg">
  <source src="page2.jpg">
  <source src="page3.jpg">
</picture>
```
<hr />
```ruby
openseadragon_picture_tag 'page1.jpg'
```

```html
<picture data-openseadragon="true">
  <source media="openseadragon" src="page1.jpg" />
</picture>
```
<hr />
```ruby
openseadragon_picture_tag Openseadragon::Image.new(id: 'xyz', width: 1234, height: 5678)
```

```html
<picture data-openseadragon="true">
  <source media="openseadragon" data-openseadragon="{ identifier: 'xyz', width: 1234, height: 5678, scale_factors: ...}" />
</picture>
```