start/stop scripts based on pid file + documentation of spark.executor.uri parameter in local.conf.template

Very minor but handy update of the start/stop scripts, I needed this in order to start/stop the job server from a supervision tool (we use monit):

added the parameter PIDFILE to local.sh.template
server_start.sh now refuses to start if a process with a pid matching the content of $PIDFILE is running
added server_stop.sh
mentioned server_stop.sh in README
Also, it took me some time to realise how to plug the job server to a spark package present in HDFS when running in "development mode" ("sbt re-start") => an example with explanation is now present in local.conf.template.