Add mad_std function
Add fwhm2sigma function
Move stats to extern
Add sigma_to_fwhm function
Prepend "gaussian_" to FWHM <-> sigma conversion functions
Updates to extern.stat funcs
Hardcode mad_std constant
Merge pull request #209 from larrybradley/stats

Add new mad_std and fwhm2sigma stat functions
Restructured main doc page
Minor doc changes
Add new overview page
Add new installation page
Add new getting started page
Add new background page
Remove width/height attributes for banner logo
Put user doc link directly in main doc page
Add in-development API note
Centralize dependency warnings
Add PSF Reference/API
consistent doc indentation [skip ci]