Linux compatibility and EXIF data fixes

When trying the script on my photos I discoved three issues :

1) In some EXIF, date fields can be set to 0000:00:00 and this is not managed by the script. I detect this case and fallback to next date detection method.

2) The stats command was returning Block size and not a file date with %B flag. I changed it to %Y to get last modification date for Linux system.

3) When checking for existance of some Date EXIF tags, there was a typo in the tag name, it was ending with "d" in the  if condition, but was without "d" when getting the tag from the map.