Docs: PHPDoc used to check for missing docs

- install-date.php documentation enhanced
- /bin/parse-api-doc.sh checks for missing docs
Security: Remove meta generator from HTML.

In the header you found a meta-information which WordPress-Version is used.
This makes it easy for attackers to determine the weak points of your system.
To enhance the security the generator information will be removed from HTML.
Tests: Script that runs all tests