added bayesian blocks binning for the craic

Stumbled across bayesian blocks and thought it'd be a cool feature to add to triangle.py. I've managed to make it function, although it relies on astroML being installed, which in turn relies on scikit-learn being installed. Could do a subtree job on it instead down the line. I tested it out on demo.py and got this:

http://broiler.astrometry.net/~kilian/bb_demo.png

which looks a little funky. Don't know if it's to do with the default priors or what, reading into Scargle et al. to learn more.

I don't know what one really writes in a pull request.