Prevent multiple "bootups" from occuring if the class is loaded twice

So this is based off your current tag, but not the latest commit.  The error I'm getting with your current tag is "Fatal error: Class 'Normalizer' not found in .../vendor/patchwork/utf8/class/Patchwork/Utf8/bootup.php on line 292" because the autoloader of my app (Laravel 4/Composer) is loading utf8 twice, once for the app and once for the package I'm working on in the Laravel workbench.

I tried your most recent commit, but I get a "Fatal error: Class 'Normalizer' not found in /Users/reinhard/Work/BKWLD/Open Source/Laravel 4 Packages/vendor/patchwork/utf8/class/Patchwork/Utf8/bootup.php on line 292" during the composer install.

This commit **does** fix my issue.