Avoid needing native Node.js modules when building for browser

[jonnor]

The NPM packages for

• https://github.com/noflo/noflo-image
• https://github.com/noflo/noflo-canvas
• https://github.com/noflo/noflo-ccv
  not only uses native C++ Node.js modules (requring a C++ compiler and all that), but also non-standard dependencies to be installed outside of NPM. This makes it a pain to setup, especially on Mac / Windows.
  These dependencies are only needed when running on Node.js (not for browser) - so we should find a better way of dealing with them.

Split out of #9