Compose module for nginx.

This module allows compose several subrequests into one reply.  The
same thing may be achieved by SSI, but unlike SSI this module will also
set Content-Length and allow range requests.

Basically it grabs X-Compose and X-Compose-Length headers from original
reply and does the following:

    1. If X-Compose-Length present, sets Content-Length to it's value.
    2. Composes multiple subrequests from X-Compose headers.

When used without X-Compose-Length it's basically identical to SSI and
not needed.  The X-Compose-Length header must be calculated correctly by
somebody.

To compile nginx with compose module, use "--add-module <path>" option to
nginx configure.