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:

*Can be passed as a set of all sequence ids in the order to be plotted.*

to:

*Must be an iterable containing all sequence IDs in the alignment.*