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.
Merge pull request #371 from TooTallNate/update/add-fs-readstream-support

node: ability to .attach() a `fs.ReadStream` instance