Add FASTQ splitting by sample id to qiime.util

This is needed for qiita.  @antgonza and @gregcaporaso suggested adding the functionality to QIIME instead, so here it is!
I can use qiime.format.format_fastq_record inside of this function, but this function is still needed to actually write to an output file (unless there's another library function in QIIME I'm not aware of that already does that, but I suspected not since the ```write_seqs_to_fasta``` function defined in this file was in use)