bracketing in m: tag files with +/- ev

[Marsu42]

Problem: When doing Canon or ML bracketing in M mode (like you're supposed to :-)), the files don't contain the ev spacing tag like Av/Tv files do ("AEB Bracket Value" in exiftool). This is a pity since this is very useful to display/sort hdr source images (like in Lightroom) or to use hdr software that use this value, if it's missing you have to specify it manually (like in Photomatrix).

Suggestion: Add an xmp sidecar to the cr2 that specifies the bracketing value used.