readme: tell people to use alpha-r2748 instead of release-1.5.27.1-beta
system: roll back fd optimization
Add google-sparehash include paths.
Disable critical image beaconing in system tests. With this enabled image inline tests don't work, since only critical images will be inlined.
Add Timer to constructor of NgxRequestContext.
copy-includes: make suggested messages understand tags
copy-includes: copy some headers that don't end in .h
psol: removing psol
psol: document having to download psol
system-test: critical css and lazyload_images-with-beaconing

Squash-merge of Jud's #303
merge commit
copy-includes: merge commit
copy-includes: merge commit
Merge branch 'master' into trunk-tracking
Merge branch 'trunk-tracking' of github.com:pagespeed/ngx_pagespeed into trunk-tracking
copy-includes: typo
readme: installation instructions
readme: on some systems no .zip suffix