Added convenience methods for accessing best fit and the corresponding SED.

* Add a number of methods to mbb_fit_results related to the best fit.
  ** The best fit is stored on initialization.
  ** mbb_fit_results.get_best_fit gets it, .best_fit_chisq gets the chisq,
     .best_fit_sed gets the SED corresponding to the best fit.
* Added a bunch of class method documentation.
* Changed __repr__ and __str__ for modified_blackbody to handle
  None values of uneeded parameters (like alpha if noalpha=True).
Merge branch 'master' of https://github.com/aconley/mbb_emcee

Conflicts:
	mbb_emcee/likelihood.py
	mbb_emcee/mbb_fit.py
	mbb_emcee/modified_blackbody.py