merged branch fabpot/extension-registration (PR #917)

This PR was merged into the master branch.

Commits
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4487387 reverted the early registration of extensions (extensions are now loaded as before -- as late as possible) -- closes #910

Discussion
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reverted the early registration of extensions (extensions are now loaded as before -- as late as possible)

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by stof at 2012-11-30T20:18:51Z

Looks good to me

diff --cc lib/Twig/Environment.php
Simple merge