Made background transparent, added resize to prototype

[cfreeley]

Not sure if you accept PRs, and this is a subjective one, but it made developing my game with this program easier.

• Took advantage of the canvas alpha option to make the background (0 squares) transparent.
• Includes the example resize() code to the base class

[bitplanets]

I think would be useful for others.

[seiyria]

I did this in my fork as well: https://github.com/seiyria/mixel for those looking for something to just use.