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 git submodules. If we publish it to npm, it will be easier to reuse this code and more incentive to contribute to its development. 

You can build for the browser with `browserify index.js --standalone blockhashjs >
blockhash.js` and test it with the following html:

```html
<!DOCTYPE html>
<html>
  <head>
    <title>Blockhash</title>
  </head>
  <body>
    <script src="node_modules/png-js/zlib.js"></script>
    <script src="blockhash.js"></script>
  </body>
</html>
```

```javascript
var bits = 16;
var blockhash = blockhashjs.blockhash;
blockhash('test.jpg', bits, 1, function(error, result) {
 console.log('hash: ' + result);
});
```

You can also run tests via `npm run test`.

Please publish this repo via NPM. :+1: