Initial change to use an num for units to display

NodaTime's PeriodUnits as a parameter to specify which units to display.
GetRelativeTime overloads have been modified to remove the countWeek
parameter (defaults still doesn't display weeks).
Probably needs more unit tests, but as the heavy lifting is done by
NodaTime, I'm not too nervous about it.
Merge pull request #8 from Pvlerick/master

Initial change to use an num for units to display