Implement http_brotli_static module

Today, nginx supports the http_gzip_static module (http://nginx.org/en/docs/http/ngx_http_gzip_static_module.html) for doing ahead-of-time gzip compression of assets; this is very useful because it allows using high-ratio/low-performance compressors like Zopfli.

Web Browsers (Chrome and Firefox, to start) are about to start supporting a new Content-Encoding, named Brotli, which offers MUCH improved compression ratios but with slow performance. http://textslashplain.com/2015/09/10/brotli/ As such, we need nginx to support a similar module (e.g. http_brotli_static) for this new content-encoding.