Use multipart post for upload

[jo]

The library currently encodes the image inline as base64 in the post data.
It would be better to send a multipart/related request instead.

One way to do it would be to add an upload method, eg:

module Openphoto
  require 'net/http/post/multipart'

  class Client
    def upload photo, params = {}
      params["photo"] = UploadIO.new(File.open(photo), "image/jpeg", File.basename(photo))
      access_token = prepare_access_token
      request = Net::HTTP::Post::Multipart.new('/photo/upload.json', params)
      access_token.sign! request
      url = URI.parse @hostname
      response = Net::HTTP.start(url.host, url.port, :use_ssl => true) do |http|
        http.request request
      end
      response.extend Openphoto::Response
      response
    end
  end
end

(used in https://github.com/jo/trovebox-uploader)

One could also enhance the connect method.