[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.
When you create a `SparkConf` it picks up `spark.jars` anyway, so we shouldn't set it again here. For Windows, `getAddedJars` actually does some post-processing on the jar paths, so if we set it again here then `sys.props("spark.jars")` and `sc.getConf.get("spark.jars")` will be inconsistent.