Data science 20141007 rc

Revisions based on previous Pull Request: https://github.com/jmmastey/level_up_exercises/pull/138

-break out `CONFIDENCE_LEVEL` and `PROBABILITY_THRESHOLD` into constants
-print out the clear winner or warning that there is no clear winner
-changed Dataset tests to check three sample files; winner a, winner b, no clear winner
-removed unused `total_sample_size` method
Guard clauses didn't seem as straightforward and readable at first, but I'm really starting to like them!