Make the alias method numerically stable

[jcazevedo]

This PR makes the alias method numerically stable, as recommended here. It fixes match errors in cases such as the following:

scala> discrete((0 until 300).map(_ -> 10.0 / 3.0):_*)
scala.MatchError: List() (of class scala.collection.immutable.Nil$)
        at probability_monad.Distribution$$anon$19.alias(Distribution.scala:281)
        at probability_monad.Distribution$$anon$19.<init>(Distribution.scala:291)
        at probability_monad.Distribution$.discrete(Distribution.scala:270)
        at .<init>(<console>:17)
        at .<clinit>(<console>)
        at .<init>(<console>:7)
        at .<clinit>(<console>)
        at $print(<console>)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:601)
        at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:734)
        at scala.tools.nsc.interpreter.IMain$Request.loadAndRun(IMain.scala:983)
        at scala.tools.nsc.interpreter.IMain.loadAndRunReq$1(IMain.scala:573)
        at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:604)
        at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:568)
        at scala.tools.nsc.interpreter.ILoop.reallyInterpret$1(ILoop.scala:745)
        at scala.tools.nsc.interpreter.ILoop.interpretStartingWith(ILoop.scala:790)
        at scala.tools.nsc.interpreter.ILoop.command(ILoop.scala:702)
        at scala.tools.nsc.interpreter.ILoop.processLine$1(ILoop.scala:566)
        at scala.tools.nsc.interpreter.ILoop.innerLoop$1(ILoop.scala:573)
        at scala.tools.nsc.interpreter.ILoop.loop(ILoop.scala:576)
        at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply$mcZ$sp(ILoop.scala:867)
        at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:822)
        at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:822)
        at scala.tools.nsc.util.ScalaClassLoader$.savingContextLoader(ScalaClassLoader.scala:135)
        at scala.tools.nsc.interpreter.ILoop.process(ILoop.scala:822)
        at scala.tools.nsc.interpreter.ILoop.main(ILoop.scala:889)
        at xsbt.ConsoleInterface.run(ConsoleInterface.scala:57)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:601)
        at sbt.compiler.AnalyzingCompiler.call(AnalyzingCompiler.scala:73)
        at sbt.compiler.AnalyzingCompiler.console(AnalyzingCompiler.scala:64)
        at sbt.Console.console0$1(Console.scala:23)
        at sbt.Console$$anonfun$apply$2$$anonfun$apply$1.apply$mcV$sp(Console.scala:24)
        at sbt.TrapExit$.executeMain$1(TrapExit.scala:33)
        at sbt.TrapExit$$anon$1.run(TrapExit.scala:42)