Add .accept() in similar fashion to the .type() method.

Hey TJ

Here is something I've wanted to do for so long, I keep using .accept() only to be told it's not there.

```
request.get('/agent')
        .accept('json')
        .end(callback);
```

As opposed to:

```
request.get('/agent')
        .set('Accept', 'application/json')
        .end(callback);
```

With services being more REST aware I find myself resorting to this more often.

I first checked in a rebuild of the superagent.js file, some changes from before were outstanding and I needed a clean slate.

Tests added and all found to be working. Updated history and tagged (although this may not reflect in a simple pull request) for new release.

Following up with the docs...