Add an optional closure parameter to HadoopRDD instantiation to used when creating any local JobConfs.
Remove the new HadoopRDD constructor from SparkContext API, plus some minor style changes.
Merge pull request #52 from harveyfeng/hadoop-closure

Add an optional closure parameter to HadoopRDD instantiation to use when creating local JobConfs.

Having HadoopRDD accept this optional closure eliminates the need for the HadoopFileRDD added earlier. It makes the HadoopRDD more general, in that the caller can specify any JobConf initialization flow.