Improve in-line documentation
Rename `Chart#mixin` to `Chart#attach`

The name "mixin" incorrectly implies state sharing, which this API has
been designed to avoid. The name "attachment" more clearly communicates
the intended use: to associate the `draw` function of one chart which
contains another.
Add assertion for lifecycle event names

Because Layers emit a pre-defined set of event names, d3.chart can help
developers prevent mistakes by asserting the name passed to `Layer#on`
and `Layer#off` is part of that set.
Release v0.2

Increment version, rebuild distribution files, and update examples.