[SPARK-1090] improvement on spark_shell (help information, configure memory)

https://spark-project.atlassian.net/browse/SPARK-1090

spark-shell should print help information about parameters and should allow user to configure exe memory
there is no document about hot to set --cores/-c in spark-shell

and also

users should be able to set executor memory through command line options

In this PR I also check the format of the options passed by the user