Allow the first argument of Predictor#predict(date) to be omitted to imply today.
Write tests.
Require assert in tests.
Add underscore dependency.
Add initial implementation with Bells#currentPeriod() working.
Add _bells as an abbreviation of bellScheduleRepresentation.
Clean up implementation file.
Implement Bells#nextBell(now).

On the creation of a Bells object, a sorted array of all the bells in a
given bell schedule is created for efficiency in #nextBell().
Adjust documentation and tests to have #nextBell() return a string instead of a Moment.
Optimize validation in Bells#constructor() to use _.isObject().
Ensure there is always a return value in a _.bellsFindKey predicate.
Clarify documentation of date/time argument format.