Fixes a bug and add support for returning the time

* When calculating the next() or previous() valid entry, the module wouldn't "backtrack" enough, and might miss a sooner crontab entry than was found. This has been fixed.
* Added the ability to return the timestamp of the future event, instead of just the delta (pass delta=False to .next() or .previous())
* Added a method to test a given timestamp or datetime against a Crontab entry
* Special thanks to Manish Dubey github.com/mdubey for the pull request that spawned these features, provided new tests, and prompted the discovery of the bug