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
Sorry, scratch the idea of allowing subsets: since majority consensus is plotted on the x-axis, it isn't so simple to allow a subset of sequences b/c majority consensus may change depending on the subset. Let's leave `sequence_order` as a strict reordering.

Reordering (including subsets) could be a good addition to the `Alignment` API in the future (e.g., a `take` method). We probably don't want to duplicate that functionality here in the plotting method.