readme: fix debian dependencies

As @bnoordhuis points out, `build-essential` depends on `make` and older
releases call `git` as `git-core`.
Merge pull request #159 from pagespeed/jefftk-debian-dependencies

readme: fix debian dependencies
shutdown-fix: fix a possible crasher on shutdown
Merge pull request #160 from pagespeed/oschaaf-shutdown-fix

shutdown-fix: fix a possible crasher on shutdown
readme: document tengine usage
Merge branch 'master' into oschaaf-message-handler

Conflicts:
	src/ngx_pagespeed.cc
message-handler: add psol version in log entries