Date interval provider

[gburgett]

We should have an interval provider that can handle dates. It should accept in the constructor a Locale and a TimeZone object, with alternate constructors for the default locale and time zone. It should also accept a calendar field number, representing the length of the interval.

We can use Calendar.roll() in order to get the next intervals, and should be able to get and parse the names using a SimpleDateFormat. This could also be a constructor arg.