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 add a description below this line of what the x- and y-axes are? i.e., y-axis is labeled by sequence ID, x-axis is labeled by majority consensus.