readme: recommend tmpfs for file cache
readme: use release
messages: unbreak /ngx_pagespeed_messages

The shared circular buffer wasn't hooked up fully, which meant loading
/ngx_pagespeed_messages didn't work.  This fixes that and adds a test.

I also noticed while adding this that the 'Handling of large files' test
wasn't set up properly, so I converted that to use start_test.

Fixing that exposed another bug where the 'Handling of large files' test
was actually failing but being marked as an expected failure by being
grouped in with the test above.  Adding `pagespeed MaxHtmlParseBytes 5000`
to the appropriate location made it test what it was supposed to be testing
again, and the underlying feature wasn't broken.
Merge pull request #560 from pagespeed/jefftk-fix-messages

messages: unbreak /ngx_pagespeed_messages
system-tests: Test keepalive behaviour after a 304 response
Merge pull request #567 from pagespeed/oschaaf-304-timeout

system-tests: Test keepalive behaviour after a 304 response
Valgrind: Add an automated test

This makes nginx run in the background under valgrind,
with both a master and a child process.
Valgrind errors will be redirected to `valgrind.log`.
When `USE_VALGRIND` is set, all system tests will be run under valgrind,
and at the end a new test is appended which ensures no valgrind errors
where encountered.

It is also worth noting that:
- There is a new file, `valgrind.sup`, which contains a few suppressions.
- Some tests behave flakey under valgrind. For now these are appended
  to the expected failures (when under valgrind only).
- 'Possibly lost' errors are all suppressed to get the amount of false
  positives manageable.
Merge pull request #565 from pagespeed/oschaaf-valgrind

Valgrind: Add an automated test
valgrind: suppressions file might not be in current directory
valgrind: unflake cache purging test

Fixes #569.