refs #6257 visitorUUID equal by default for each new Piwik.Tracker
refs #6257 Piwik.getVisitorId() function
refs #6257 getVisitorId() unit tests
fix possible typo: undefined variable $head
Ignore any opcache_invalidate errors

Some webhosters with opcache enabled might disable functions like this for security reasons. This should prevent foreign applications from deleting cached things.

Without ignoring this error piwik is unusable and will always shows the error message:
```
There is an error. Please report the message (...)
Warning: Zend OPcache API is restricted by "restrict_api" configuration directive in /piwik/core/CacheFile.php on line 219
```

This call should treat the same way as opcache_reset() in core/Filesystem.php added with faffbd68b41a00a0e220138285094f517c07778d

Other functions which might be disabled:
- opcache_reset
- opcache_get_configuration
- opcache_get_status
- opcache_compile_file
Merge pull request #6300 from KoffeinKaio/master

Proxy piwik.php script does not send http headers
Merge pull request #6303 from stackcoder/patch-1

Ignore any opcache_invalidate errors
Add silent fail in front of opcache_
refs #6204 wondering if archiving still works when removing this dependency from Actions to Contents plugin. The dependency to the event plugin would be still there. It might work now as I changed Contents plugin to use its own action type recently
refs #6204 ignore content actions
Merge pull request #6310 from piwik/6204

Remove contents plugin dependency from actions plugin
Fix xhprof build in Gentoo.
refs #6257 removed getVisitorId() from Piwik API
Merge pull request #6311 from sabl0r/gentoo

aclocal and autoconf calls are necessary to build the xhprof php extension
Merge pull request #6258 from a4tunado/6257

refs #6257 visitorUUID equal by default for each new Piwik.Tracker
refs #6258 adding the minified piwik.js
Merge pull request #1 from piwik/master

updating the latest patches

#stuff