* standby: add standby mode
In standby mode PageSpeed is off, except it serves .pagespeed. resources and
PageSpeed query parameters are interpreted. This is equivalent to "off" in
mod_pagespeed.
With this change, "off" in mod_pagespeed is deprecated, and people should use
"standby" instead.
* add test file
* rewrite doc
* get tests to pass under memcached
* doc: minor display tweak