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
@gregcaporaso how do you feel about having the default legend labels as `Minimum`, `Median`, `Maximum`? I think those labels better reflect their placement on the legend. Or we could default to not labeling the colorbar, since users will probably always need to provide more meaningful labels.