"Masking" is an outdated browser trick/hack, it should be avoided in modern times.
Login at GitHub and add the domain to your Pages site.
Then login at Go Daddy and modify the DNS for the domain.
A records. (control the domain http://yourdomain.com )
CNAME record (control the sub-domain http://www.yourdomain.com
@Nate What would I need to do in order to enable https on github. Currently I have domains from namecheap pointing to github and I have no problem enabling https. But for the domain on GoDaddy github wont allow me to enable https in the options. It says, "Enforce HTTPS — Unavailable for your site because you have a custom domain configured".
@bobbidigi, The IP addresses have changed to allow for CDN and SSL.
Make sure to edit the A records for host @ to point to the new IP addresses. This can take a day or two to update. Then within your GitHub account remove the custom domain and re-add it.