Issue description

While packaging emacs-25.1 I noticed that we still keep the old (deprecated) emacs package sets (emacs25Packages) around.

I think that now is the right time to get rid of the deprecated emacs25packages along with a cleanup of the emacs packaging in all-packages.nix (note that I still refer to emacs25 in these comments):

• Remove all instances of emacs25Packages and emacsPackages
• Rename emacsPackagesNg and emacs25PackagesNg to emacsPackages and emacs25Packages
• Rename the emacs-24 folder to just emacs (drop the version-extension)

Additionally:

• Adapt consistent naming scheme for emacs25Macport_24_5
• Get rid of emacs25Packages and emacs25 completely in favor of just emacsPackages and emacs? (as we don't need to keep emacs25 around)
• Introduce emacs_humongous (similar to vim_humongous) with all (even unstable) features enabled