Created a plot() method that generates a heatmap from a DissimilarityMatrix 

In relation to  issue #684
Since this is a new feature that happened after scikit-bio 0.2.1, please move this under `## Version 0.2.1-dev` (you'll need to add a `### Features` heading there too).

`skbio.stats.distance._base.py` is private, so we'll want to note the API addition that users will see. Please update the text to read:

```
Added ``plot`` method to ``skbio.stats.distance.DissimilarityMatrix`` for creating basic heatmaps of a dissimilarity/distance matrix (see [#684](https://github.com/biocore/scikit-bio/issues/684)). Also added  ``_repr_png_`` and ``_repr_svg_`` methods for automatic display in the IPython Notebook, with ``png`` and ``svg`` properties for direct access.
```