Documentation: Encourage use of reduceByKey instead of groupByKey.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #784 from pwendell/group-by-key and squashes the following commits:

9b4505f [Patrick Wendell] Small fix
6347924 [Patrick Wendell] Documentation: Encourage use of reduceByKey instead of groupByKey.
Package docs

This is a few changes based on the original patch by @scrapcodes.

Author: Prashant Sharma <prashant.s@imaginea.com>
Author: Patrick Wendell <pwendell@gmail.com>

Closes #785 from pwendell/package-docs and squashes the following commits:

c32b731 [Patrick Wendell] Changes based on Prashant's patch
c0463d3 [Prashant Sharma] added eof new line
ce8bf73 [Prashant Sharma] Added eof new line to all files.
4c35f2e [Prashant Sharma] SPARK-1563 Add package-info.java and package.scala files for all packages that appear in docs
fix different versions of commons-lang dependency and apache/spark#746 addendum

Author: witgo <witgo@qq.com>

Closes #754 from witgo/commons-lang and squashes the following commits:

3ebab31 [witgo] merge master
f3b8fa2 [witgo] merge master
2083fae [witgo] repeat definition
5599cdb [witgo] multiple version of sbt  dependency
c1b66a1 [witgo] fix different versions of commons-lang dependency
HOTFIX: Don't build Javadoc in Maven when creating releases.

Because we've added java package descriptions in some packages that don't
have any Java files, running the Javadoc target hits this issue:

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4492654

To fix this I've simply removed the javadoc target when publishing
releases.