Merge commit 'fa8ea95d' from PR #953

This pull request fixes the broken zoom functionality (issue #611) and
console errors in the drawing demo. It simply adds the
willDestroyContextMyself attribute, and removes the now unimplemented
functions in the defaultInteractionModel. Comments have also been added
to explain to users why calls are missing.

> Hi, yep, the bug is still there. Select the zoom functionality in
> https://dygraphs.com/gallery/#g/drawing with the console open, and try
> a zoom, or move the mouse over the canvas.