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!
Actually... this is not easy, since it introduces a circular import (in qiime.util, I have to import ```format_fastq_record``` from qiime.format, but qiime.format has to import ```load_qiime_config``` from qiime.util).  No matter what I do, this will cause an ```ImportError```.  So I suggest leaving this as-is...