Implementation of plus, uplus, minus, uminus for datetimes and durations

Currently:

>> t = datetime()
                t =

                2018-10-08 18:12:41

>> dt = hours(1.5)
                dt =

                1.5 hr

>> t - dt
error: minus method not defined for datetime class
>> t + (-dt)
error: uminus method not defined for duration class
>> t + t
error: plus method not defined for datetime class
>> t + (-t)
error: uminus method not defined for datetime class
>> +t
error: uplus method not defined for datetime class

I might even get a merge request to you to have this done soon-ish. ish.