Add "import scalaz.stream._" to sbt's initialCommands

This PR adds `import scalaz.stream._` to sbt's `initialCommands`, see http://www.scala-sbt.org/0.13.2/docs/Howto/scala.html#initial for documentation of this setting. This means that `scalaz.stream._` is automatically imported when entering the Scala REPL via sbt's `console` task. This is very handy when playing around with the library in the REPL, since one does not need to type this import statement again and again.