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
Change:

*If None, defaults to the order in which they were passed; 0,1,2...etc.*

to:

*If None, sequences are plotted as they are ordered in the alignment.*