Remove Exiv2 warnings, add user stories.
US28: Show a help message
US29: Add a switch for the output bit depth

- Add a switch "-b" to select the output bit depth: 16, 24 or 32.
- Add a properties dialog to select the desired output properties.
- Add the tranlations for all new strings.
- Fix the save action must be disabled on start.
- Fix AboutDialog was not in the hdrmerge namespace.