HTTP status messages move: OutputPage -> HttpStatus

HTTP status messages move: OutputPage -> HttpStatus

OutputPage::getStatusMessage() is a method to convert a numeric HTTP status
code to an english message.  It does not really belong to the OutputPage were
it was for historical reason.
This patch move the basic function to a non MediaWiki dependant class in our
directory includes/libs.  We could potentially enhances it, but I do not see
any use cases for us yet.
I have renamed the method to the shorter 'getMessage' since the word 'status'
is now in the class name.

Summary of changes:
* OutputPage::getStatusMessage becomes HttpStatus::getMessage
* Method moved to the new includes/libs/HttpStatus.php
* Autoloader updated
* History kept by using 'svn copy'
* No functional changes
* No input/output format changes
* Old occurences modified in phase3
* OutputPage::getStatusMessages() marked as deprecated

PHPUnit test suite is fine.
Tested manually using a redirection