Kernel#sleep

This is so small I'm embarrassed about having my name in AUTHORS, so I'll gladly back it out if there are any qualms.  The behavior described [here](http://www.ruby-doc.org/core-1.9.3/Kernel.html#method-i-sleep) includes a zero-arg call that sleeps forever for which there is no test case.  I wasn't sure how best to accomplish this without introducing Threads or some such.