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
I think there's a bug in the original recipe. It should be plotting the first and last sequence IDs, and every 3rd label (I think?) in between, but the labels are actually being plotted from 1, 2, 3, ... Can you please fix and add a unit test for this case?