Whitelisting publicly readable title with regex

This patch make it possible to whitelist pages which anonymous users may
see. It is similar to $wgWhitelistRead expect it uses regular
expressions, the list of regex are to be added in the new global array
$wgWhitelistReadRegexp.

This would be useful in a semi-public team wiki situation where the
admin would want to hide an entire namespace from everyone except those
in a particular group due to sensitive team specific information.

Added new unit test testUserCan in includes/TitleTest.php to
test this new functionality.

* adds $wgWhitelistReadRegexp to DefaultSettings.php
* updates RELEASE-NOTES-1.21 new features
* updates CREDITS

Signed-off-by: Antoine Musso <hashar@free.fr>

Change-Id: I83f6a614874c3d289ff4bd8d015f1d9c92e500b6