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
Use this link instead:

http://matplotlib.org/examples/color/colormaps_reference.html

Also add:

*If ``None``, defaults to the colormap specified in the matplotlib rc file.*