caching: support for proxy_cache

Add support for proxy_cache integration with pagespeed with:

a) Handling cache-hit-header to bypass content handling by pagespeed.
b) pagespeed_test_conf.template with the proxy_cache configuration blocks.
c) System tests for checking hit and miss headers and purge requests.
d) Checking for proxy_cache_purge module during start-up for tests.

Squash-merge of Anupama's #404
Fix paths for files moved from apache/ to system/
Add CFLAGS to fix OpenSSL linker error.
Use SystemRewriteDriverFactory::InitStats instead of RewriteDriverFactory::InitStats
Fix add_instrumentation test.
Remove unnecessary cc sources from config.
Revert removing system/ cc files, as that causes linker errors.
Merge branch 'jud-trunk-tracking' of https://github.com/pagespeed/ngx_pagespeed into jud-trunk-tracking