[SPARK-1897] Respect spark.jars (and --jars) in spark-shell

Spark shell currently overwrites `spark.jars` with `ADD_JARS`. In all modes except yarn-cluster, this means the `--jar` flag passed to `bin/spark-shell` is also discarded. However, in the [docs](http://people.apache.org/~pwendell/spark-1.0.0-rc7-docs/scala-programming-guide.html#initializing-spark), we explicitly tell the users to add the jars this way.

Author: Andrew Or <andrewor14@gmail.com>

Closes #849 from andrewor14/shell-jars and squashes the following commits:

928a7e6 [Andrew Or] ',' -> "," (minor)
afc357c [Andrew Or] Handle spark.jars == "" in SparkILoop, not SparkSubmit
c6da113 [Andrew Or] Do not set spark.jars to ""
d8549f7 [Andrew Or] Respect spark.jars and --jars in spark-shell