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

In relation to  issue #684
If the backend isn't set to `Agg`, Travis will fail when trying to execute any matplotlib code, complaining that `DISPLAY` isn't set. I've also noticed those warnings in the notebook, though everything seems to display fine.

I'm not sure of the best way to solve this -- it seems like others are using the `Agg` trick to work around it (e.g., see [here](http://stackoverflow.com/questions/21321292/using-matplotlib-when-display-is-undefined)). It'd be great to get rid of those warnings though...