enable kernel version check by default
preserve config file on upgrades
fixed: set-default got triggered when volume is not initialized
removed redundant umount handling and unused variables
added sleep time to read messages in debug mode
add BTRFS_ROLLBACK_DIALOG to disable rollback dialog on boot (optional), closes #9
set debug mode sleep time to 0 by default
added some notes for GRUB 2.02, closes #11
suppress info message from btrfs device scan
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Merge pull request #12 from visit1985/master

Fixes for #9, #11 and some other improvements