statistics: check that statistics work and are not extrnally accessible

* Add tests for statistics.
* We weren't increasing resource_404_count on 404s.
  * This required giving `NgxBaseFetch` a `NgxServerContext` pointer.
* /ngx_pagespeed_statistics was publically available.
* Made a pass over the readme to add a new configuration option and fixed up
  serveral things that were out of date while I was there.

Fixes #248.
Merge pull request #249 from pagespeed/jefftk-now-sw-have-stats

statistics: check that statistics work and are not externally accessible