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
Ignore the part of this comment dealing with subsets (see my comment [here](https://github.com/biocore/scikit-bio/pull/779/files#r21430911))