[Fix #204] Update out-dated comments

This PR is self-explanatory.

Author: Andrew Or <andrewor14@gmail.com>

Closes #381 from andrewor14/master and squashes the following commits:

3e8dde2 [Andrew Or] Fix comments for #204
[SPARK-1403] Move the class loader creation back to where it was in 0.9.0

[SPARK-1403] I investigated why spark 0.9.0 loads fine on mesos while spark 1.0.0 fails. What I found was that in SparkEnv.scala, while creating the SparkEnv object, the current thread's classloader is null. But in 0.9.0, at the same place, it is set to org.apache.spark.repl.ExecutorClassLoader . I saw that https://github.com/apache/spark/commit/7edbea41b43e0dc11a2de156be220db8b7952d01 moved it to it current place. I moved it back and saw that 1.0.0 started working fine on mesos.

I just created a minimal patch that allows me to run spark on mesos correctly. It seems like SecurityManager's creation needs to be taken into account for a correct fix. Also moving the creation of the serializer out of SparkEnv might be a part of the right solution. PTAL.

Author: Bharath Bhushan <manku.timma@outlook.com>

Closes #322 from manku-timma/spark-1403 and squashes the following commits:

606c2b9 [Bharath Bhushan] Merge remote-tracking branch 'upstream/master' into spark-1403
ec8f870 [Bharath Bhushan] revert the logger change for java 6 compatibility as PR 334 is doing it
728beca [Bharath Bhushan] Merge remote-tracking branch 'upstream/master' into spark-1403
044027d [Bharath Bhushan] fix compile error
6f260a4 [Bharath Bhushan] Merge remote-tracking branch 'upstream/master' into spark-1403
b3a053f [Bharath Bhushan] Merge remote-tracking branch 'upstream/master' into spark-1403
04b9662 [Bharath Bhushan] add missing line
4803c19 [Bharath Bhushan] Merge remote-tracking branch 'upstream/master' into spark-1403
f3c9a14 [Bharath Bhushan] Merge remote-tracking branch 'upstream/master' into spark-1403
42d3d6a [Bharath Bhushan] used code fragment from @ueshin to fix the problem in a better way
89109d7 [Bharath Bhushan] move the class loader creation back to where it was in 0.9.0
SPARK-1480: Clean up use of classloaders

The Spark codebase is a bit fast-and-loose when accessing classloaders and this has caused a few bugs to surface in master.

This patch defines some utility methods for accessing classloaders. This makes the intention when accessing a classloader much more explicit in the code and fixes a few cases where the wrong one was chosen.

case (a) -> We want the classloader that loaded Spark
case (b) -> We want the context class loader, or if not present, we want (a)

This patch provides a better fix for SPARK-1403 (https://issues.apache.org/jira/browse/SPARK-1403) than the current work around, which it reverts. It also fixes a previously unreported bug that the `./spark-submit` script did not work for running with `local` master. It didn't work because the executor classloader did not properly delegate to the context class loader (if it is defined) and in local mode the context class loader is set by the `./spark-submit` script. A unit test is added for that case.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #398 from pwendell/class-loaders and squashes the following commits:

b4a1a58 [Patrick Wendell] Minor clean up
14f1272 [Patrick Wendell] SPARK-1480: Clean up use of classloaders
[SPARK-1415] Hadoop min split for wholeTextFiles()

JIRA issue [here](https://issues.apache.org/jira/browse/SPARK-1415).

New Hadoop API of `InputFormat` does not provide the `minSplits` parameter, which makes the API incompatible between `HadoopRDD` and `NewHadoopRDD`. The PR is for constructing compatible APIs.

Though `minSplits` is deprecated by New Hadoop API, we think it is better to make APIs compatible here.

**Note** that `minSplits` in `wholeTextFiles` could only be treated as a *suggestion*, the real number of splits may not be greater than `minSplits` due to `isSplitable()=false`.

Author: Xusen Yin <yinxusen@gmail.com>

Closes #376 from yinxusen/hadoop-min-split and squashes the following commits:

76417f6 [Xusen Yin] refine comments
c10af60 [Xusen Yin] refine comments and rewrite new class for wholeTextFile
766d05b [Xusen Yin] refine Java API and comments
4875755 [Xusen Yin] add minSplits for WholeTextFiles
Enable -feature in scalac to emit language feature warnings
Quiet scalac warnings about language features by explicitly importing language features.