adjust coding style some more
Bump up file version, if we use a compression that the old code couldn't handle

This will make it abort with a sensitive error, i.e. that the file has been created by a newer version of the software.
Default compression is stored in StorageInfo

The user can set the default compression method, when she creates the storage. This method
will be used for backup unless the user overrides it with a command-line switch. That way
she can still choose for each backup.

suggested by dragonroot