trunk-tracking: update from r3924 to r3934

There aren't actually any changes required from r3924 to r3934 but the admin
work from previous versions wasn't completely ported before.

This change:
 * adds support for /pagespeed_admin and /pagespeed_global_admin
 * removes NgxPagespeedConsoleAsyncFetch using NgxBaseFetch instead
 * adds options StatisticsPath, GlobalStatisticsPath, ConsolePath,
   MessagesPath, AdminPath, and GlobalAdminPath which allow site owners to
   control what path these are served under
 * changes /ngx_pagespeed_statistics etc from default-on to default-off,
   requiring explicit path configuration to function
 * does not fix the problem where trunk-tracking fails tests on 1.5 post #653
Merge pull request #666 from pagespeed/jefftk-update-trunk-tracking

trunk-tracking: update from r3924 to r3934