Validate TTL

This adds support (and tests) for validating the user supplied TTL. This closes #1. Though you recently closed that issue as wontfix, I do think it is a good idea to validate the TTL as it is relied upon in other tasks (deletion, for example).

I don't code in sh much, but I did my best to ensure that this code is portable and matched conventions in your code.

Tests are included, though I have not tested it in a production environment.

---Alex