Added USE_I18N setting, which lets you turn off internationalization overhead...

Added USE_I18N setting, which lets you turn off internationalization overhead with a single setting. Defaults to True. Currently only affects the admin i18n JavaScript, but I'll be adding other optimizations.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3247 bcc190cf-cafb-0310-a4f2-bffc1f526a37