* Use mock canvas object replacing `canvas`
Fixes#3515.
A mock canvas object is added to the unit tests, which makes usage of module `canvas` voluntary.
The issue with `canvas` is that it requires an external dependency to `cairo`. This complicates setting up a develop environment for `vis.js`
- Removed `canvas` from `package.json`
- Added section to README.md with instructions on how to install `canvas` instead.
* Removed debugger statements
* Updates for review
* Fixes for review