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
I don't understand what this check is accomplishing: `sequence_order` is an iterable and `sequence_count` is the number of sequences in the alignment. Is it necessary to compare them here?