Parallel fetching of HTTP resources through MIME multipart

This package's BatchClient applies standard MIME multipart encoding to HTTP
messages, providing a standards-conservant technique for making parallelizable
HTTP requests over a single proxy connection.

To make a batch request, open a new request on a BatchClient instance and add
your subrequests, along with callbacks that will receive the subresponses.
Once all parallelized requests are added, complete the request; the request is
made and the batched subresponses are provided to your callbacks.