adds net/json

This adds a short hand for json requests. The API is rather short:

```js
json( "url/to/json", succes, [error] );
```

with `error` being  optional. Also, It adds the doc for `request` which I have forgotten so far.

I wasn't so sure about the naming. I stuck with `json`. Do you think that is too short. `loadJSON`, `getJSON`, ... What do you think?