Adds a plotting method for the distance matrix in heatmap form.

Related to issue #684
Changed the way the heatmap title is made

Before it used ax.set_xlabel(), but in testing I found that the xlabel is difficult to find and it would be much simpler to use the ax.set_title() function and orient it to the bottom of the graph.
Created unit tests for the plot() method

Tests check the xtick and ytick labels, and the sanity of the output, regarding the title and amount of axes, and whether or not it returns a Figure. Also I was unable to find a way to retreive information regarding the cmap input. Help would be much appreciated.
Addresses @jairideout 's comments
Addresses @jairideout 's comments
Fix pep8 errors.
Addresses further @jairideout 's comments
Fixes TravisCI errors.
Fixed 'no module named 'six'' error
Fixes error 2nd time
Rewrote the "Examples" section of the docstring
Merge branch 'master' into heatmap

Conflicts:
	skbio/stats/distance/tests/test_base.py
Fixed flake8 whitespace error.
Whitespace error
Added my 'plot' method to the changelog.
Addresses @jairideout 's comments
Merge pull request #751 from Kleptobismol/heatmap

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