client: add basic `field()` and `attach()` functions

Just really basic wrappers around the HTML5 `FormData` object.
Merge pull request #373 from TooTallNate/add/client-form-data

client: add basic `field()` and `attach()` functions
node: ability to .attach() a `fs.ReadStream` instance

This allows us to do, i.e.:

``` js
request.post('/upload')
  .attach('file', fs.createReadStream('foo.txt'))
  .end(fn);
```

Eventually it would be better to add support for arbitrary
`stream.Readable` instances, but this is good enough for now.