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'
```
Can we call this `_pretty_print_strs`? It took me a couple seconds to figure out what it was supposed to do.