Added an ExportFastq command.

ExportFastq takes an ADAM / BAM / SAM file as input, and produces a collection of fastq files
for the single-ended and paired-end reads with those file(s).  Depending on the number of partitions,
it may produce multiple fastq files; these can then be concatenated as necessary.
Created Export command, adding ADAM to BAM export.

Wrote an 'unconvert' method in SAMRecordConverter.
Adding TODO comments.