Support `FormData`

Posting FormData can be done using something like:

``` js
var fd = new FormData()
fd.append('image', file)
var req = new XMLHttpRequest()
req.onload = function () {
callback(null, req.responseText)
}
req.onerror = callback
req.open('post', 'https://api.imgur.com:443/3/image.json')
req.setRequestHeader('Authorization', 'Client-ID ' + clientID)
req.send(fd)
```

However, I can see know way of handling `FormData` using the built in `http` module.

(see https://github.com/substack/node-browserify/issues/439)