Refactor to use npm modules instead of git submodules.

Hi, this PR sets up the project to be published via npm by using npm modules for dependencies instead of submodules. If we publish it to npm, it will be easier to reuse this code and more incentive to contribute to its development. Next step is to build for browser using the browser version of `png.js` and  `zlib.js`.

You can also run tests via `npm run test`.