Use the elm-html style function for style attributes

Currently, rather than using the `style` function, the `attribute` function is being used for style attributes.

currently 

``` html
<div style="background-color: red;"></div>
```

outputs as

```
div [ attribute "style" "background-color: red;" ]
    []
```

but

``` elm
div [ style [("background-color", "red") ]
    []
```

would be much nicer
