Enable unit tests with ScalaTest

Created by: cviebig

This is the pull request to introduce ScalaTest framework to the common project repository as suggested by @aalexandrov in https://github.com/TU-Berlin-DIMA/IMPRO-3.SS14/issues/1

A test spec to try this setup out could be:

package de.tu_berlin.impro3.scala.classification.randomforest

import org.scalatest._

class TreeSpec extends FlatSpec with Matchers {

  "A tree" should "always be happy!" in {
    true should be (true)
  }
}