[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.
Hm yeah. Actually for this PR I think I'll just modify only the jar paths added to the classpath of the shell, but not worry about changing the value of `spark.jars` when different things are set.