Add OrdResFormatError
Merge branch 'master' into issue-645

Conflicts:
	skbio/io/__init__.py
	skbio/io/_exception.py
Move ordination results test data into skbio/io/tests/data

These files need to be renamed.
Rename ordres test data files to be prepended with 'ordres_'.
Move ordination results test data files into skbio/io/tests/data

I forgot some from the previous move.
Initial pass at OrdinationResults reader

Includes passing unit tests (more need to be added still).
Add tests for reading invalid files

Also fixed a few error messages that weren't clear.
Port ordination results writing code and tests
Add ordres sniffer and tests

Also refactor some of the parsing code (still more to-do).
Add OO read/write API to OrdinationResults; deprecate from_file/to_file
Add unit tests for OrdinationResults OO I/O and deprecated methods
Fix pep8/flake8 errors
Added skbio.stats.ordination.assert_ordination_results_equal

Had this logic duplicated in two different unit test locations.
Add changelog mention for ordres I/O refactoring
Add more details to ordres format spec.
Refactor ordination results writer

Less duplication of code and literals, better decomposed.
Refactor biplot parser into coord parser

Removes a lot of duplicate code.
Refactor vector section parsing

Further duplicate code removal.
Refactored parser's length checking code
Add invalid ordres file that ends where a blank line is expected