chart.unlayer and chart.relayer

Fixed `chart.unlayer` (it was referencing `_layerList` which we weren't using.) Unlayer also returns the layer that was just removed rather than a reference to the chart.
Added `chart.relayer` that takes a `name` and a `layer` object to be re-added.
Added tests for both.

Questions:
1. Should we just modify `layer` to do the re-adding of a layer? If we only have 2 arguments, we can treat it as a relayering.

We should probably bump to 0.2 since this will add a new api method. I haven't made those changes yet to package.json or changelog.md.