Add stacking sample

Adds a small sample of stacking column series, with snippet.

The update to example.html removes the code dump at the end of the
sample, as this was causing clutter, and did not serve to show usage as
easily as the snippets.
Add "Zoomable" sample

Adds a chart using the App Store "number of new apps per month" data,
and enables zooming and panning on it.