[ADAM-463] AvroMultiParquetRDD loads multiple Parquet files

[ADAM-463] Added AvroMultiParquetRDD

First, updated the FileLocator to include a childLocators method, and added its implementation in all the child FileLocator implementations.

Next, added an HDFSFileLocator implementation. 

Finally, added AvroMultiParquetRDD, which takes a FileLocator which indicates the _parent_ directory (in HDFS, or S3) of a set of parquet files, and which loads _all_ row groups from all the parquet files that are immediate children of that FileLocator.
        
(Joint work with Carl Yeksigian.)

How are credentials handled here? Wouldn't you need to pass the credentials at some point in time to list the items in each bucket?