Ahc 1.7.x - Grizzly, support an easy way to send multipart body through a BodyGen

It aims to provide a way to stream the multipart file upload body and to keep a low memory consumption

This is related to this discussion:
https://java.net/projects/grizzly/lists/users/archive/2013-08/message/32

Here's a code snippet that shows how to use it
https://gist.github.com/slorber/6368183