[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

Author: CodingCat <zhunansjtu@gmail.com>

Closes #599 from CodingCat/spark_shell_improve and squashes the following commits:

de5aa38 [CodingCat] add parameter to set driver memory
915cbf8 [CodingCat] improvement on spark_shell (help information, configure memory)
SPARK-1098: Minor cleanup of ClassTag usage in Java API

Our usage of fake ClassTags in this manner is probably not healthy, but I'm not sure if there's a better solution available, so I just cleaned up and documented the current one.

Author: Aaron Davidson <aaron@databricks.com>

Closes #604 from aarondav/master and squashes the following commits:

b398e89 [Aaron Davidson] SPARK-1098: Minor cleanup of ClassTag usage in Java API