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
Good idea re: the default. I've found that this parameter is really important for heatmaps as it can be essential for getting visibly labels (but maybe if we call ``seaborn.heatmap`` within this method, that parameter wouldn't be necessary anymore).