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.
You should probably rename BundleChecker to BundleCollector (BundleChecker is misleading, it's not "fsck" implementation) and move it to external source files.
Something like bundle_collector.cc/hh.