Added heatmap plotting method to Alignment (and tests).

Fixes #765

Based on @gregcaporaso's scikit-bio cookbook recipe:

http://nbviewer.ipython.org/github/biocore/scikit-bio-cookbook/blob/master/Alignment%20viewing%20and%20filtering.ipynb
Can you test that `legend_labels` contains exactly three elements (at the top of the method), and raise an error if not? Also add a unit test for this case.