Updating Oval to use new styles

This isn't quite ready yet. I've only found one problem (which really means two problems since in the meta issue #711 we plan to change the way style specs work )

The issue is in setting oval styles via the style method:

```ruby
Shoes.app do
  style(Shoes::Oval, fill: green)
  oval 10, 10, 50
end
```
produces a black oval. I haven't figured that out yet, but figured I'd commit what I have and stick it up here until next chance I get to look at this.