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.
Starting to write an 'unconvert' method in SAMRecordConverter
Working on unconverter, created Export command.
Fleshed out the initial, local version of BAMExport.
Adding TODO comments.