added Mod operator and a nice example with isEven test

mytest/Even.scala

src/main/scala/leon/Evaluator.scala

src/main/scala/leon/FairZ3Solver.scala

src/main/scala/leon/plugin/CodeExtraction.scala

src/main/scala/leon/plugin/Extractors.scala

src/main/scala/leon/purescala/PrettyPrinter.scala

src/main/scala/leon/purescala/Trees.scala