Added simple way of jumping to current buffer's class/function definitions

Added simple way of jumping to current buffer's class/function definitions

The new `python-nav-jump-to-defun' function allows a user to jump fast
and easy to a function or class definition in the current buffer. For
this a entry in the menu and the C-c C-j keybinding have been
introduced.

New functions:
 + `python-nav-list-defun-positions'
 + `python-nav-read-defun'
 + `python-nav-jump-to-defun'

The new function `python-nav-list-defun-positions' does the same as a
relevant part of the `python-imenu-create-index' so the latter has
been refactored to use it.