add range header support for client.download

[suconghou]

I am using your go-dropy and it is quite well with

io.Copy(os.Stdout, client.Open("/demo.txt"))

I write the data to the http.ResponseWriter , it work well , broswers can directly download the file ,
but it is a pity that the response is not rangeable

would you please modify the https://github.com/tj/go-dropbox/blob/master/client.go#L51 and add another parameters such as a map for addon http request header , so that the response can be rangeable .

It is very usefull sometimes , please

thank you very much !