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 like listing all attributes because most of them are optional and it's useful to see which ones are present or not. It might not be clear that an attribute is missing if it is omitted from the output because you'd have to print a couple different objects and compare their output (e.g., printing objects with and without `site`s). Since this is a `__str__` method, it might not be reasonable to assume that the user has read the docstring describing its behavior. Does that make sense?