Transitionable: Fix doc; Make API more consistent

* delay() should really return this.

Why?

I should be able to do something like this:

```javascript
var t = new Transitionable(0);
t
.set(1, {duration: 100})
.delay(200)
.set(2, {duration: 100});
```

Currently this is not possible, since delay does not return this.

* Fix API (minor improvements)