updated sublime-text ebuilds to be more generic & some minor fixes

1. I included a useflag "multislot" to make installing eselect-sublime optional
2. I changed the source URI's of the tarballs of major version 3, the old ones wouldn't download
3. I made the ebuilds a little more generic by extracting the major version from $PV (when the upstream guys stop changing the naming scheme of their tarballs this should enable simply copying the ebuild and change the version suffix to update it)

thanks for considering this :)