Add I/O recipe

Fixes #28. @gregcaporaso @ebolyen can you please review? [Link](http://nbviewer.ipython.org/github/jairideout/scikit-bio-cookbook/blob/bb987e2e0d53442fb83d7b388f96aa18163a1d1c/reading-and-writing-files-in-scikit-bio.ipynb) to rendered notebook.