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
It might be handy to allow this to be a subset of the sequences, so that users can filter and/or reorder sequences in the plot (may be useful for alignments with large #s of seqs). @gregcaporaso thoughts?