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 mention the behavior I described above, i.e. that all, or a subset of, the sequence IDs can be provided? Probably important to note that these are expected to be sequence IDs.

Also describe what happens in the default case (i.e., all sequences are plotted in the order that they are present in the alignment).