Correctly handle 0 in --older_than, --most-recent and --delete-older-than

This comes from #208.

Now previous month can be snapshotted as well as optimized (October is included in November):

```
curator λ ./curator/curator_script.py -n --host 127.0.0.1 snapshot --older-than 0 --time-unit months --prefix statistics-super-fast- --timestring %Y%m --repository ceph_s3 --snapshot-name whatever
2014-11-04 22:08:31,366 INFO      Job starting...
2014-11-04 22:08:31,366 INFO      DRY RUN MODE.  No changes will be made.
2014-11-04 22:08:31,391 INFO      DRY RUN: Capturing snapshots of specified indices...
2014-11-04 22:08:31,505 INFO      statistics-super-fast-201411 is within the threshold period (0 months).
2014-11-04 22:08:31,505 INFO      DRY RUN: Snapshot will capture indices: statistics-super-fast-201310, statistics-super-fast-201311, statistics-super-fast-201312, statistics-super-fast-201401, statistics-super-fast-201402, statistics-super-fast-201403, statistics-super-fast-201404, statistics-super-fast-201405, statistics-super-fast-201406, statistics-super-fast-201407, statistics-super-fast-201408, statistics-super-fast-201409, statistics-super-fast-201410
2014-11-04 22:08:31,506 INFO      DRY RUN: Snapshots captured for specified indices.
2014-11-04 22:08:31,506 INFO      Done in 0:00:00.162480.
```