review of fasta docs

@jairideout, this is ready for review and merge. Overall, the docs look excellent. One thing I changed through-out is how to re-load the ``StringIO``. I thought that re-instantiating the object from a string (``fs`` for *fasta string* in the updated code) is more similar to how the code will be used, since users are generally opening a file if trying to apply the examples to their own data. In other words, I think ``open`` is conceptually more similar to ``StringIO`` than ``StringIO.seek``, so just calling ``StringIO`` again seems more clear to me. 

fixes #708