Modify README slightly
Add smarter generation of initial parameters.  Interface clean up.

* mbb_fit objects can now generate their own initial parameter values
  taking limits and fixed parameters into account.
  ** As part of this, which parameters are fixed has been moved
     from likelihood to mbb_fit.
* Much interface cleanup.  Try to avoid mbb_fit and mbb_fit_results
  accessing _ elements of like, so add a bunch of interface
  routines up and down the chain.
* Version number bumped to 0.1.3
Add ability to directly specify luminosity distance.

* Add lumdist argument to get_lir, get_agn, get_dustmass.
  z is still required to get limits.
* run_mbb_emcee argument for setting lumdist.
* Fix extra \n in __str__ for mbb_fit_results