“Dance Nation” at Playwrights Horizons is Clare Barron’s explosive and raw look at a ragtag troupe of 11-to-13 year-old competitive dancers as they discover their bodies, their power, and their ambition amid the glorious horror of adolescence.  Both messy and explicit, this hilarious and unsettling new play is refreshingly weird and thrillingly honest, featuring an excellent ensemble cast and perfect direction.  Catch this one if you can.