IO exists!

**not ready for merge**

Basic framework for the IO subpackage. Still need to add docstrings.
I think that ``parse_fasta`` and ``parse_fastq`` should still be exposed at this level, even if they raise deprecation warnings, since they're so widely used (e.g., the precursor to ``parse_fasta``, cogent's ``MinimalFastaParser``, was the most frequently imported functionality from cogent in QIIME).