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
Optimized imports

Optimized imports and arranged according to scala style guide @
https://cwiki.apache.org/confluence/display/SPARK/Spark+Code+Style+Guide#SparkCodeStyleGuide-Imports

Author: NirmalReddy <nirmal.reddy@imaginea.com>
Author: NirmalReddy <nirmal_reddy2000@yahoo.com>

Closes #613 from NirmalReddy/opt-imports and squashes the following commits:

578b4f5 [NirmalReddy] imported java.lang.Double as JDouble
a2cbcc5 [NirmalReddy] addressed the comments
776d664 [NirmalReddy] Optimized imports in core
[SPARK-1105] fix site scala version error in doc

https://spark-project.atlassian.net/browse/SPARK-1105

fix site scala version error

Author: CodingCat <zhunansjtu@gmail.com>

Closes #616 from CodingCat/doc_version and squashes the following commits:

eafd99a [CodingCat] fix site scala version error in doc
Revert "[SPARK-1105] fix site scala version error in doc"

This reverts commit d99773d5bba674cc1434c86435b6d9b3739314c8.
SPARK-1106: check key name and identity file before launch a cluster

I launched an EC2 cluster without providing a key name and an identity file. The error showed up after two minutes. It would be good to check those options before launch, given the fact that EC2 billing rounds up to hours.

JIRA: https://spark-project.atlassian.net/browse/SPARK-1106

Author: Xiangrui Meng <meng@databricks.com>

Closes #617 from mengxr/ec2 and squashes the following commits:

2dfb316 [Xiangrui Meng] check key name and identity file before launch a cluster