Fixes #138, disallow special characters for repository name

Fixes #138 
Furthermore repos with special characters cannot be deleted.

The class FileNameAttribute can be deleted (only usage). If Regex check the filename there can't be any characters (depends on Regex) which are disallowed in path.
FileNameAttribute should stay to handle errors if the regex will extended unwary.

The message for other Languages should be updated. Resource-Name is 'Validation_FileName_Regex'