which-function-mode is a global mode
Use ~/.emacs.d/site-lisp for backported code
Add python-mode from Emacs 24.4 (upcoming version)

There's a performance issue in python-mode bundled with 24.3 when using
imenu or which-function-mode.

Bug report: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15295

Upstream commit: 443c0b2b3f90624bc254af8e758ee19dbe772c9a