Fix --underscore file renaming. It breaks when special characters or bad encoding is used in file names.

[DanielLukic]

Right now the rename operation will fail with an error message. Tagomatic will continue processing files. This is OK for now.

But the implementation could be improved by:

• Trying FileUtils.mv first, then only use 'system mv'.
• Doing a two step rename: First cleaning up the name - replacing all 'unwanted' characters. Then doing the underscore removal.

Overall I do not like this --underscore option very much so far.