Updated save-pixels signature to match get-pixels. Fixes #6

[twolfson]

As discussed in #6, it would be ideal to have consistent signatures between get-pixels and save-pixels. To keep consistency with Canvas, we are using [width, height, channels] for our ndarray's. In this PR:

• Upgrade to get-pixels@3.0.1 to pickup latest signature
• Updated README.md with new ndarray signature
• Updated save-pixels.js to work with new shape

[mikolalysenko]

Thanks!