[SPARK-7155] [CORE] Allow newAPIHadoopFile to support comma-separated list of files as input

See JIRA: https://issues.apache.org/jira/browse/SPARK-7155

SparkContext's newAPIHadoopFile() does not support comma-separated list of files. For example, the following:
```scala
sc.newAPIHadoopFile("/root/file1.txt,/root/file2.txt", classOf[TextInputFormat], classOf[LongWritable], classOf[Text])
```
will throw
```
org.apache.hadoop.mapreduce.lib.input.InvalidInputException: Input path does not exist: file:/root/file1.txt,/root/file2.txt
```
However, the other API hadoopFile() is able to process comma-separated list of files correctly. In addition, since sc.textFile() uses hadoopFile(), it is also able to process comma-separated list of files correctly.

That means the behaviors of hadoopFile() and newAPIHadoopFile() are not aligned.

This pull request fix this issue and allows newAPIHadoopFile() to support comma-separated list of files as input.

A unit test has also been added in SparkContextSuite.scala. It creates two temporary text files as the input and tested against sc.textFile(), sc.hadoopFile(), and sc.newAPIHadoopFile().

Note: The contribution is my original work and that I license the work to the project under the project's open source license.

Author: yongtang <yongtang@users.noreply.github.com>

Closes #5708 from yongtang/SPARK-7155 and squashes the following commits:

654c80c [yongtang] [SPARK-7155] [CORE] Remove unneeded temp file deletion in unit test as parent dir is already temporary.
26faa6a [yongtang] [SPARK-7155] [CORE] Support comma-separated list of files as input for newAPIHadoopFile, wholeTextFiles, and binaryFiles. Use setInputPaths for consistency.
73e1f16 [yongtang] [SPARK-7155] [CORE] Allow newAPIHadoopFile to support comma-separated list of files as input.