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

In relation to  issue #684
Can you add an `Examples` section below the `Returns` section here? See `OrdinationResults.plot` for an example of how to format these (the plots will show up in our built documentation too, e.g. see [here](http://scikit-bio.org/docs/latest/generated/generated/skbio.stats.ordination.OrdinationResults.plot.html)).