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
Please add a blank line after the summary line, and then dedent the following two lines so that they line up with the start of the block quote (`"""`). Can you also make these descriptions complete sentences and describe how the y-axis is labeled? i.e., not all sequence IDs are plotted.