Move file upload responsibility

[arnoFleming]

At the moment we use the client object to add items to a board or transfer (here we'll use a board as an example):

@client = WeTransfer::Client.new(api_key: "foo")
board = @client.create_board( ... )
@client.add_items(board: board)  do |builder|
  builder.add_file(name: File.basename(__FILE__), io: File.open(__FILE__, 'rb'))
  builder.add_file_at(path: fixtures_dir + 'Japan-01.jpg')
  builder.add_web_url(url: 'http://www.google.com', title: 'google')
end

or file uploads but we should make the transfer or board responsible for file upload, not the client.

@client = WeTransfer::Client.new(api_key: "foo")
board = @client.create_board( ... )
board.add_items do |builder| 
  builder.add_file(name: File.basename(__FILE__), io: File.open(__FILE__, 'rb'))
  builder.add_file_at(path: fixtures_dir + 'Japan-01.jpg')
  builder.add_web_url(url: 'http://www.google.com', title: 'google')
end

Please note that there are tests that are skipped because we don't want the original behaviour -- you should change and then implement -- search for the string "this interface is still experimental" to see them.