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
`aa` stands for amino acid. In the original cookbook recipe, this plotting function was only used for protein sequences, but it's been generalized to work with any character set. Can you update this variable name to something more general, e.g., `char_`?