[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`.