Feature/checkpoint : add the possibility to do live backups

[benoitc]

See http://rocksdb.org/blog/2609/use-checkpoints-for-efficient-snapshots/ for more infos

[benoitc]

this change extract the checkpoint patch from #11

[mocchira]

@benoitc Thanks as always.

I'd like you to

1. Remove the commit bump rocksdb...
2. Add a test case to confirm whether iterator operations also works properly on a db created by Checkpoint
3. delete checkpoint instead of checkpoint = NULL;

[benoitc]

@mocchira done

• added iterator test
• fixed C code to use delete
• rebased.

[mocchira]

LGTM. Thank you for your contribution.