support PIL installation with easy_install where we have no PIL subdirectory

This is a fixup of the regression introduced by e693d6cf (#16).

If PIL is installed with easy_install, there is no PIL subdirectory, thus we should import like "import Image". But if PIL is installed without easy_install, there is a PIL subdirectory, thus we should import like "from PIL import Image".

This pull request will support both cases.

Please see https://mail.python.org/pipermail/image-sig/2010-August/006480.html for the detail of the issue.