move basic model functionality into sourcemodel classes
import extinction module from specutils
WIP on branch model-refactor
WIP on model-refactor
WIP on model-refactor
WIP on model-refactor
WIP on model-refactor
update relevant files for v0.4 development
Merge branch 'model-refactor'

Conflicts:
	sncosmo/snanaio.py
WIP: Updated fitting, plotting to work with refactored models
Update setup stuff to astropy 0.3 methods
refine guessing t0, amplitude in fit_lc
WIP refactoring models
Fix bug in obsmodel names, fit result improvements
mainly doc improvements
add missing import in plotting
fix bug in fitting, improve docs
automatic t0 bounds in nest_lc
Flexible options in read_snana_fits
add guess_z option to fit_lc()