node.js crypto: Changed HMAC and Hash params to `any`

1. `createHmac` also accepts Buffers as the `key` param, not only strings
2. `Hmac.digest` and `Hash.digest` both return a Buffer object, if no encoding is specified, so also not just strings.

Docs:
http://nodejs.org/api/crypto.html

So we'll stick with 

```
digest(encoding: 'buffer'):Buffer;
digest(encoding: string): any;
digest(): Buffer; 
```

? I like this as well.