Can't change fps without making a timeline

I'm considering an API addition that allows:
```python
anim = amp.Animation([block], fps=fps)
```
where ```fps``` gets passed to the constructor of Timeline when no timeline is passed.

Another option is to allow ```Timeline``` to accept an integer for ```t``` so one would do:
```python
anim = amp.Animation([block], Timeline(len(block), fps=fps))
```

cc @TomNicholas Thoughts?