pluggable autodomain via Domainer object

Now that #531 is merged in, we can actually do #455. This branch introduces the `Domainer` object.

```
var xScale = new Plottable.Scale.Linear()
             .setDomainer(new Plottable.Domainer().pad().nice());
```

This lets you turn on and off common settings for autodomaining, and gives us a place to put more of these options (include 0, etc.).

Also, you can set your own function if you want. Here's the default one:

```
new Plottable.Domainer((extents: any[][]) => [d3.min(extents, (e) => e[0]), d3.max(extents, (e) => e[1])])
```

This means that `scale.padDomain()` and `scale.nice()` are removed, you must go though a `Domainer` object.
i feel like this jsdoc could be clearer, like giving more context on the fact that the extents are set by the renderers and an example of why you might want to override this function