[SPARK-1919] Fix Windows spark-shell --jars

We were trying to add `file:/C:/path/to/my.jar` to the class path. We should add `C:/path/to/my.jar` instead (see changes in `SparkILoop.scala`). Tested on Windows 8.1.
Maybe we should deprecate ADD_JAR if you can add JARs with `--jars` now. Not sure why we left it in there. But maybe we can complain in the shell script and just leave it out.