forward-sexp-function: Set to nil in python-mode.

The default behavior of python-mode is very strange and inconsistent,
as it considers the last line of a method to be "the end" of the method
and moving backwards a sexp there will not move backwards over a symbol,
but backwards to the beginning of the method. The maintainer considers
this a feature and thinks setting `forward-sexp-function` to nil is
the way to fix it for users.

Elpy does this now, but it required a new function to be run in the
python hook.

See http://debbugs.gnu.org/db/13/13642.html for the report and
discussion.
elpy-initialize-variables: Bind RET as no-op in ac-completion-map

When using auto-complete as a completion library for "search as you
type" style completion, using RET usually means the user is happy
with the current completion and just wants a newline, much like
SPC means the user is happy with the current contents and wants
a space inserted. The default behavior of ac uses RET to confirm
the current completion, which can be very annoying.

This commit changes this default behavior.