SPARK-1658: Correctly identify if maven is installed and working

The current test is checking the exit code of "tail" rather than "mvn".
This new check will make sure that mvn is installed and was able to
execute the "version command".

Author: Rahul Singhal <rahul.singhal@guavus.com>

Closes #580 from rahulsinghaliitd/SPARK-1658 and squashes the following commits:

83c0313 [Rahul Singhal] SPARK-1658: Correctly identify if maven is installed and working
bf821b9 [Rahul Singhal] SPARK-1658: Correctly identify if maven is installed and working
SPARK-1663. (Addendum) Fix signature of one version of JavaPairRDDStream.reduceByKeyAndWindow()

It appears that one of these methods doesn't use `org.apache.spark.api.java.function.Function2` like all the others, but uses Scala's `Function2`.

Author: Sean Owen <sowen@cloudera.com>

Closes #633 from srowen/SPARK-1663.2 and squashes the following commits:

1e0232d [Sean Owen] Fix signature of one version of reduceByKeyAndWindow to use Java API Function2, as apparently intended
SPARK-1703 Warn users if Spark is run on JRE6 but compiled with JDK7.

This add some guards and good warning messages if users hit this issue. /cc @aarondav with whom I discussed parts of the design.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #627 from pwendell/jdk6 and squashes the following commits:

a38a958 [Patrick Wendell] Code review feedback
94e9f84 [Patrick Wendell] SPARK-1703 Warn users if Spark is run on JRE6 but compiled with JDK7.
SPARK-1629. Addendum: Depend on commons lang3 (already used by tachyon) as it's used in ReplSuite, and return to use lang3 utility in Utils.scala

For consideration. This was proposed in related discussion: https://github.com/apache/spark/pull/569

Author: Sean Owen <sowen@cloudera.com>

Closes #635 from srowen/SPARK-1629.2 and squashes the following commits:

a442b98 [Sean Owen] Depend on commons lang3 (already used by tachyon) as it's used in ReplSuite, and return to use lang3 utility in Utils.scala
SPARK-1693: Most of the tests throw a java.lang.SecurityException when s...

...park built for hadoop 2.3.0 , 2.4.0

Author: witgo <witgo@qq.com>

Closes #628 from witgo/SPARK-1693_new and squashes the following commits:

e3af968 [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1693_new
dc63905 [witgo] SPARK-1693: Most of the tests throw a java.lang.SecurityException when spark built for hadoop 2.3.0 , 2.4.0
make master support multiple executors per worker
doc update
doc update
style fix