readme: move testing to a wiki page
memcached-threads: use only one

See https://code.google.com/p/modpagespeed/issues/detail?id=672
Merge pull request #274 from pagespeed/oschaaf-memcached-threads

memcached-threads: use only one
readme: we don't depend on external mod_pagespeed binaries
Use ngx_thread_system in NgxRewriteDriverFactory.
readme: google group -> mailing list; link to announce
readme: tighten it up
readme: move build troubleshooting to a wiki page
readme: minor reorg
readme: better link to build-from-source
Merge pull request #277 from pagespeed/jud-nginx-thread

Use ngx_thread_system in NgxRewriteDriverFactory.