Move subpackage-specific exceptions into their respective subpackages.
DOC: update skbio.util docstring
DOC: Add mention of API changes to changelog.
fix pep8 issue
Fix checklist.py error
DOC: add link to #577, thanks @gregcaporaso!
Merge pull request #581 from jairideout/issue-577

Move exception classes into their respective subpackages/submodules