Update import.php to work around a bug in Apple's AddressBook

Apple's AddressBook separates groups (CATEGORIES) in exported vCards by escaped commas instead of unescaped ones as defined in the RFC. 

The bug results in groups with a trailing backslash being added to owncloud, when importing vCards which contain more than one group for the contact.

This is a very quick and dirty fix to unescape these commas again to avoid the issue.

The fix is only applied if the vCard was created on Mac OSX by checking the PRODID field.

The fix does not check, if the CATEGORIES field does already include unescaped commas at which state, it is probably safe to assume that Apple fixed the problem.
If someone more capable than me want's to add that additional check (my regex fu is bad), that'd be great.