Improved exception matchers to have stacktrace in case of failure

[edmondop]

As from ML discussion, the case is simple:

class FailingSpec extends Specification{

    def is = s2""""
                                    This is cool $e1      """

    def e1 =
        foreach( 1 to 10 ){
            index =>foreach (index to index * 3) {
                subIndex => subIndex must explodeBooh or throwA[NullPointerException]
            }
    }

    def explodeBooh = new Matcher[Int]{
        override def apply[S <: Int](t: Expectable[S]): MatchResult[S] = {
            val value = t.value
            if(value%2==0)
                explodingMethod
            else
                result(true,"It works","It doesn't work",t)
        }
    }

    def explodingMethod[S]:MatchResult[S] = {
        subExplodingMethod

    }

    def subExplodingMethod[S]:MatchResult[S] = {
        throw new IllegalArgumentException("baah")
    }
}

[etorreborre]

Thanks for this pull request. I incorporated as f6fe94f (and republished 2.4-SNAPSHOT)