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
In addition to checking the length of `sequence_order`, can you please ensure that `sequence_order` only contains unique IDs? i.e., no repeated IDs