Add Interval class

This still needs docs and a bit of formatting cleanup, but the tests are extensive, so it should be ready for general feedback.

Main inspirations for this were:

http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Range.html
http://en.wikipedia.org/wiki/Interval_(mathematics)

I put it in `quiver.compare` since I also want to add stuff from #60 there.  `quiver.comparables` could also work, but that might not make as much sense if Interval or other functionality works with arbitrary `Comparator`s instead of requing `Comparable`s.  `quiver.math` is another option.

I deprecated `quiver.iterables.extent(iterable)` in favor of `new Interval.span(iterable)`.