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.
Remove skbio.io import_module calls from I/O classes

Looks like this might not have been necessary b/c of the top-level skbio.io import in skbio/__init__.py.
Add ScikitBioABC class and tests

Only includes __str__ for now and basic tests. Next step is to update codebase to inherit from this class (and define __str__ methods as necessary).
update some classes to derive from abc

only updated ones that already have __str__ methods
ScikitBioABC -> skbio_object

Thanks @wasade for suggesting this new name!
skbio_object -> SkbioObject

Thanks @ebolyen for suggesting this name!
Addresses @jairideout 's comments
Addresses @jairideout 's comments
Fix ordination example file
Fix pep8 errors.
Merge pull request #752 from jairideout/abc-init

First pass at ABC
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.