#1974 - Add option to make all resizes and/or thumbs jpg.

- Added make_all_resizes_jpg and make_all_thumbs_jpg Gallery variables.
- Updated gallery_installer, module.info, and install.sql to v56 to add them
- Revised Item_Model functions that return thumb/resize paths and urls.
- Changed item::find_by_path to add type argument and accomodate new options.
- Changed file_proxy to use new item::find_by_path.
- Updated file_proxy and data_rest to return the correct mime types.
- Included functions in graphics helper to enable/disable the new options (NOT FINISHED).
- Added unit tests (NOT FINISHED).