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
Merge branch 'issue564' of https://github.com/ebolyen/scikit-bio into issue-564
Merge branch 'master' into issue-564

Conflicts:
	skbio/alignment/_alignment.py
	skbio/tree/_tree.py
First working version of dm_to_DissimilarityMatrix
Refactor dm parser to support Dissimilarity/DistanceMatrix classes
Remove redundant DistanceMatrix tests
Refactor test code, much shorter/cleaner now
Make naming consistent in tests