Makes Interval lazy

[bethard]

Makes start and end on Interval use def instead of val, and revises Interval implementations to use lazy val. The goal is to defer evaluation of things like UnknownInterval, which would otherwise cause anything that references them to throw an exception at construction time. Note that this required small changes in the test suite, because if neither start nor end is called, no exception will be produced.

I originally started down the path we had discussed for deferring this evaluation by making all the Interval operators into Interval => Intervals, but that ended up not really working out. For example, if ThisRepeatingInterval is an Interval => Interval and TwoDigitYear is an Interval => Interval, then it's not clear how to pass the TwoDigitYear as the Interval argument to ThisRepeatingInterval in a expression like "January of 89".

[awmarkley]

It seems like this solves the issue we were concerned about by protecting the parsing code from exploding during temporal operator normalization. I imagine when these temporal operators are examined for correctness we will know that we've found a relative TPZ when we hit an exception.