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
Can you remove this variable and just supply these values in the `heatmap` call below? I think these were defined separately in the original cookbook recipe because they were reused several times, but they're only used once here.