isochrones: MultiNest fitting; unit tests

Big changes to how StarModel fits happen now---fitting now defaults to using MultiNest via PyMultiNest. To take advantage of this change, you must install MultiNest and PyMultiNest. If you don't have these packages, you can follow these directions to install them.

MultiNest does a much better job than emcee of exploring the stellar model parameter space when only photometry is being used as observational constraints, because the posterior can be quite multi-modal. As a result, you must use MultiNest if you want a fair fit when you're just using photometry.

The way to fit a StarModel now is using the .fit() method, which will default to .fit_multinest() if MultiNest is available, and will otherwise default back to using emcee via .fit_mcmc(). Please let me know if you have any troubles with this.

The starfit command-line script should work the same as before; again defaulting to MultiNest if available.

In addition, I have finally included unit tests: type nosetests isochrones on the command line to run them. If all goes well, everything should be OK.