MATH-927
Made "BitsStreamGenerator" class "Serializable" to allow cloning of
subclasses. Added cloning test for all "RealDistribution" classes.
Thanks to Dennis Hendriks.


git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1428822 13f79535-47bb-0310-9956-ffa450edef68