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
Ugh. We should use whatever the user has in their config file, but let's keep this parameterizable here and we should always pass something other than ``jet`` in our usage examples...