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.
Punctuation typo.
Should be
```
 " performs import from source to destination storage;\n"
+" gc <storage path> - performs chunk garbage collection.\n"
```