Allow Emacs 24 "deftheme" mechanism.

After reading about the [deftheme](http://batsov.com/articles/2012/02/19/color-theming-in-emacs-reloaded/) mechanism for managing themes, I wanted to move past the old color-theme package, but theme-changer was also very nice. 

These patches allow theme-changer to be used with either color-theme or deftheme.