Allow different methods of generating file ids

Allow different file id generation schemes by setting a PythonOption in the apache config file. The default scheme would use the curreent random integer generator, but other schemes might include MD5 or SHA-1 hash of the uploaded file data, UUIDs, or simply the filename of the uploaded file. Some schemes (e.g. MD5/SHA-1 hash or the uploaded file name) may result in duplicate ids, in which case we would assume the file already existed.