Ported t-digest to stream-lib's environment.

This port required a few uglinesses:

- I re-implemented a bit of guava (Preconditions, Lists, that sort of thing) even though guava is listed as a test dependency for stream-lib.  It would be better to just depend on guava.

- I added Mahout math as a test dependency in order to get some special distributions for testing.  I note that stream-lib depends on colt, pretty much all of which has been subsumed into Mahout math.  In the process of sucking that code in, we found a lot of errors.  It would be better to pull in Mahout math directly.