Add test for path conflicts
Put the original path at the end in get_possible_paths

Make it try all the combinations of prefixes and suffixes first,
and then try the original path. This way if there is a directory
named the same as our file, but without the extension and/or
prefix, we don't try to open it and avoid raising IOError.

Fixes #10.
Fix #14, respect storage.prefix when looking for files

The function ``find_all_files`` ignored ``storage.prefix`` when looking
for files, which made it fail when a prefix was used in
``STATICFILES_DIRS``.