Quickselect (find Nth ordered statistic in mostly linear time)

This adds the Select class to utils, which is modeled after the Sort class, allowing Array to call into it to get the nth item in an array based on a Comparator.  It currently uses Quickselect, but could be configured to use other algorithms without breaking the API.

Array now has methods like Array.selectRanked that act much like Array.sort(), except for selection instead of sorting.

See also the commit inside the PR with tests if you want to verify correctness. I passed all the tests when I ran and I also dropped the new gdx.jar into my game where I used it to find the nearest enemy units (no bugs). In fact I mostly made this because finding new enemy targets based on distance was taking up a lot of cpu time in my game.

Eager for criticisms and suggestions.

UPDATE:
It makes a smart choice between a naive single-pass algorithm or quickselect based on the value of k. For k = min or k = max, single-pass is faster and it uses that.  Otherwise, it uses quickselect.