ENH|API: IO registry exists
STY|API: pep8 and moved IO exceptions to skbio.io
TST: fixed StringIO for python3
TST: more python3 fixes
TST: assertTrue had 2 args, moved testclasses out of setup
DOC: added documentation about io
STY: minor changes
Merge branch 'master' of https://github.com/biocore/scikit-bio into issue564

Conflicts:
	skbio/alignment/_alignment.py
	skbio/stats/distance/_base.py
	skbio/stats/ordination/tests/test_ordination.py
	skbio/tree/_tree.py
	skbio/tree/tests/test_tree.py
	skbio/util/__init__.py
DOC/ENH: loosened concept of filehandling in procedural implementations and added convenience identifier for blank files
TST: 100% branch coverage
Merge branch 'master' of https://github.com/biocore/scikit-bio into issue564
TST: fixed for python 3
API: registration now abstracts file opening
DOC: Add doctest for pwmantel
DOC: add additional doctest examples to pwmantel

Showing/describing usage of ``lookup`` and ``strict``.
Move dm-matching code into mantel

Previously was in pwmantel, but it makes sense to support matching of distance matrices in both functions. Minimal updates to tests and docs to get things passing again. Still need additional changes to unit tests, doctests, and documentation.
Move shared test data into base class

Prepping for refactoring of unit tests.
Add unit test class for _order_dms

This function is more complicated now so it makes sense to explicitly test it.
Add error-path tests for _order_dms
Remove duplicate test data in MantelTestData