Add OrdinationResults.__str__

Also update DissimilarityMatrix/DistanceMatirx.__str__ to list `repr`-ed IDs for improved readability (see [here](http://stackoverflow.com/a/2626364/3776794) for rationale).

Part of #753.
Add docstring to _pprint_strs
Merge branch 'master' into issue-753-ordination-results

Conflicts:
	CHANGELOG.md
Merge pull request #755 from jairideout/issue-753-ordination-results

Add OrdinationResults.__str__