Extend the request class with helpers for reading server vars (server())
and HTTP request headers (header()). Refactor the existing code base
to make use of these helpers, make $_SERVER a deactivated super global.
Also introduce an is_ajax() method, which checks the X-Requested-With
header for the value 'XMLHttpRequest', which is sent by JavaScript
libraries, such as jQuery.
PHPBB3-9716
This commit also ports all ascraeus tests to the new format.
Conflicts:
tests/all_tests.php
tests/network/all_tests.php
tests/request/all_tests.php
tests/request/request_var.php
tests/template/templates/includephp.html
