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
The other plotting methods have defaulted to using matplotlib's default colormap (which unfortunately is `jet` unless the user has specified otherwise in their config file). I think we should try to be consistent across the plotting methods -- @gregcaporaso do you think we should default to a non-jet colormap or continue with our strategy of using whatever's in the user's matplotlib config?