Add a new request type: MultipartRequest

[KPull]

We need to add and support a new type of request: MultipartRequest. This is important because it is usually used by web applications to upload files to a web server.

The MultipartRequest should support:

• addRequest: Adds an HTTP request to the multipart request. Remember that a multipart request is one big HTTP request that contains smaller "HTTP requests" within it.
• addAttachment: A convenience function which calls addRequest above with a GeneralRequest object that sends the given file.