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
Sorry I didn't catch this before: the default value for `legend_labels` should be changed to a tuple instead of a list (see [here](http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument) for an explanation why).