Add .npmignore

[eliot-akira]

I noticed that the node-audio package on NPM has a few unnecessary files, including test.wav which is pretty big.

This PR may not be needed if you could remove them locally before publishing next version.

For reference, I checked: https://docs.npmjs.com/misc/developers#keeping-files-out-of-your-package

.npmignore ignores .DS_Store and node_modules by default.

[ramirezd42]

cool thanks!