Interface to MCMC codes

[karllark]

Would be great if we could use the same setup with an MCMC code. One obvious choice is the 'emcee' package.
There is some work in astropy to enable this through adding the ability to add a prior to the parameter class used by all models. Currently suspended until the refactoring of the astropy.model class is done (soon). When done by astropy, add to PAHFIT.