Change progress values to be between 0 and 1

Whereas '100' is sort of an arbitrary value without context ("it's a percentage"), having progress be from 0 to 1 is more natural, and makes it simpler to wire animation up to existing APIs. For example, to animate XHR progress with the percentage API, one would write:

```js
var x = new XMLHttpRequest();
x.addEventListener('progress', function (e) {
  if (e.lengthComputable) {
    circle.animate((e.loaded / e.total) * 100);
  }
});
x.open('GET', url);
x.send();
```

After this change, we could drop the extra multiplication:

```js
var x = new XMLHttpRequest();
x.addEventListener('progress', function (e) {
  if (e.lengthComputable) {
    circle.animate(e.loaded / e.total);
  }
});
x.open('GET', url);
x.send();
```