Begins refactoring the Gantt object to use ranges
Adds chai-datetime to development dependencies
Moves date and range functions to more appropriate place
Finishes moving tests from Range into date utilities
Fixes silly spelling error
Changes subtract().unit.to to subtract().unit.from (left over from copy and paste)
Alters add and subtract functions so that they return the date operated on
Refactors Gantt object tests to use chai-datetime. Also makes them more concise