Fix date-time and preview in Windows.

- Use QDateTime instead of localtime as a more portable way of measuring time.
  The DateTime and DateTimeOriginal tags are now correctly calculated in Windows.
- Fix DngFloatWriter so that if the JPEG conversion fails, the preview is not added to the output file.
- Record last save directory in QSettings, besides the last load directory.
- Update the README.md with the features of the new version.
Fix preview rotation