Add support for node.js crypto.Hash style interface.

This PR adds support for [node.js crypto.Hash](http://nodejs.org/api/crypto.html#crypto_class_hash) style interface:

```javascript
hash = new Adler32.Hash();
hash.update('Wikipedia');
hash.digest('hex'); // returns '11e60398'
```

Handy if you have some existing code that uses the existing crypto infrastructure and you want to adapt it to use adler32 with a minimum of fuss.

Quite unrelated to this PR, but very nice implementation of adler32.  I wrote something like this about a year ago, and only got around to cleaning it up and [releasing it](https://github.com/jwalton/adler32-js) today, but yours is about five times as fast as mine.  :)