refactored constructor; moved calculation of mime/magicdescr/crc32/md5 into private methods, to keep the constructor cleaner
now ctime and mtime are nil if there is an error during calculation; every other attribute is already the same, so it was time to do this as well
ctime and mtime only get added to the hash if they are not nil
on osx, the mime type and the magic file description are now calculated by the use of the "file" command line tool. the file magic library had to many erroneous results (e.g. it can not classify .m4b or .webarchive)
may this purge any conflicts of Fsinv.options and @@options once and for all!