merged branch za-creature/patch-1 (PR #943)

This PR was merged into the master branch.

Commits
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15be13c Update lib/Twig/Autoloader.php

Discussion
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Update lib/Twig/Autoloader.php

Added the ability to register the autoloader at the top of the stack, useful in some situations.

Specifically, I need this because I want the last autoload function registered in my application to throw an exception instead of producing a class not found error, and I need *that* autoloader to be registered before twig's.