Strip * from media file names.

Some filesystems, namely the FAT variants and NTFS, don't support the ```*``` character in file names. It is possible for a user to add a media file with that character on a machine that supports it (e.g., OS X or Linux), and fail to sync media on Windows or [some Android devices](https://code.google.com/p/ankidroid/issues/detail?id=1795#c4).

This commit just adds that character to the list of illegal characters.