Remove xemacs detector, since we do not have generic-mode anyway. Typ…

…o patrol.

Switch license, improve comments

* Switch license from v3 or later to v2 or later to ensure better compatibility
  with Emacs.

* Improve comments with the help of "M-x checkdoc"

Changes done after feedback on the wishlist bug filed to get upstart-mode.el
included in the "debian-goodies-el" package, at
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=586321