Attempt at handling ETag / If-None-Match on proxied content.

I plan on using this to front a Varnish server using a lot of ESI.

It does kind of work, but... be aware, this is my first attempt at developping
a nginx plugin, and dealing with headers after having read the body was not
exactly in the how-to.

Any comment and/or improvement and/or fork is welcome.

Thanks to http://github.com/kkung/nginx-static-etags/ for... inspiration.