Improved metadata support via EXIF metadata. Since JXR is based on a TIFF container, EXIF data is read directly from the file so it always works (even with versions of libjxr that don't have the metadata reading API).
It also solves the following issues:
- Incorrect date format on saved JXR files (was saved in ISO format instead of `yyyy:MM:dd HH:mm:ss`).
- Incorrect date type setting in EXIF data: the `DateTime` tag should be updated on every save (verified by GIMP and Photoshop). Our `CreationDate` metadata is the equivalent of the EXIF `DateTimeOriginal` tag.
Closes#22
