Add /ngx_pagespeed_statistics

This adds /ngx_pagespeed_statistics. It also uses SharedMemoryStatistics instead
of the SimpleStatistics currently used, and sets them as the default. It also
adds support for tracking per server{} statistics.

Cleaning up of shared segments is disabled, because the current psol version we
link in has a problem with the ordering of nginx's events during a configuration
reload.

The referrer statistics will be ported in a separate pull.

Related to issue #133

Squash-merge of Otto's work in #176.