"help-on-symbol-at-point" feature which uses pydoc to provide help on
the symbol under point, if available.

Mods include some name changes, a port to Emacs, binding the command
to C-c C-h, and providing a more informative error message if the
symbol's help can't be found (through use of a nasty bare except).

Note also that py-describe-mode has been moved off of C-c C-h m; it's
now just available on C-c ?

Closes SF patch #545439.