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.

Example of `OrdinationResults.__str__`:

```
>>> print(str(ord_res))
Ordination results:
	Eigvals: 2
	Proportion explained: N/A
	Species: 3x2
	Site: 3x2
	Biplot: N/A
	Site constraints: N/A
	Species IDs: 'Species1', 'Species2', 'Species3'
	Site IDs: 'Site1', 'Site2', 'Site3'
```
I don't really mind, but pprint.pprint is the stdlib function to pretty
print so the name is not new.
On Nov 6, 2014 9:01 AM, "Evan Bolyen" <notifications@github.com> wrote:

> In skbio/stats/_misc.py:
>
> > @@ -41,3 +41,17 @@ def p_value_to_str(p_value, permutations):
> >          result = ('%1.' + '%df' % decimal_places) % p_value
> >
> >      return result
> > +
> > +
> > +def _pprint_strs(strs, max_chars=80, delimiter=', ', suffix='...',):
>
> Can we call this _pretty_print_strs? It took me a couple seconds to
> figure out what it was supposed to do.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/biocore/scikit-bio/pull/755/files#r19953400>.
>