Basic garbage collection support (updated)

Hey all,

I've rebased this pull request - #16 (originally by @vitalif) - over the current master. Also added gc command to usage help.

Please review and merge. If any issues appear - let me know - i'll fix it.
Then it will be probably better to move gc from ZRestore to something like ZCollector/ZSanitizer.
I don't see any direct relation between garbage collection and restore process.