Made Uploadable to work with stream wrappers

- changed DIRECTORY_SEPARATOR to '/' - Windows does not have any problem with standard slashes - most wrappers use parse_url and it expects the separator to be a slash
 - changed Validator::validatePath that it's able to skip (optionaly, it checks by default) the check of is_dir (some wrappers like Gaufrette don't support is_dir, some of them event don't support directories at all)